In the same boat as RP, I posted this elsewhere and realized it could be applicable to more people. The subject is not putting constraints on what a relationship is allowed to become, written to a married woman who was afraid of saying "I love you" to her new female lover because she had no intention of committing to the woman in the same way as to her husband and didn't want her husband to feel replaced.
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The key thing here is communication, which, luckily, it seems like you all are quite good at. As long as your husband knows that "I love her" doesn't mean "I am planning to run off with her" or "You are no longer as pivotally important to my emotional and practical life as you once were" and as long as she knows that "I love you" doesn't mean "I am going to forsake other commitments to be with you" or even "I am capable of committing to you beyond the commitments I've already made", then you should be just fine.
Of course, the idea that love can be fine doesn't mean you should rush it. It may well be that what you have right now is a very close, loving friendship of the kind you might have with a "best friend", that also happens to involve sex. If it's not "romantic" that is perfectly fine and that sort of loving friendship is wonderful in my book. But that doesn't mean that at some point you won't find yourself doodling her name in the margins of your notebook with a heart in place of a dot over the "i", if you get what I'm saying... it might evolve into something that feels romantic, something that makes you burn to hold her close and whisper "I love you" and take her out to a fancy restaurant and hold hands all night and stare deeply into each other's eyes.
If the day comes that you realize you feel that and you want that, and your reaction is gut-wrenching fear, you may do something you'll deeply regret later, like cut her out of your life.
And the thing is, that fear is completely unnecessary. My gf and I first traded ILU's about 6 months into our relationship. It was spontaneous and really special and I won't ever forget it... and it didn't change one single thing about her relationship with her husband. He is still her life partner, they're still on track with the life they planned. I am not her life partner, and we have no plans for that to change (though neither have we closed the door on it some day being a possibility... more on that later).
What does love mean in that sort of context, what does commitment mean?
To me, loving her as "more than a friend" means I have a strong emotional reaction to her joy and her distress, I delight in surprising her and caring for her, I consider her before I consider most people in my life, I think of time with her as something that I won't do without if there's any alternative at all, and I wanna kiss her and hold her and touch her and such.
Commitment for us actually means something very similar to what you posted regarding your agreements with your friend/lover, that we tell each other about things and consider each other carefully. For me it also goes beyond that into having made a personal commitment to sticking with this and supporting her until/unless she wants to end it or life events should push us apart (somebody gets a dream job in Australia, for example). We have also both demonstrated a commitment to making time for each other in our lives (including alone time, which is no mean feat for her with a job, husband, new baby, and an active social life!). We haven't expressed much in the way of concrete commitments (x evenings together per month for example) because it hasn't seemed necessary and because our lives are a bit too chaotic for that right now.
Our relationship is still quite young to my mind... 2+ years, but unlike a mono couple might have done by now we haven't moved in together or spent exorbitant amounts of time together, so I feel like it's taken longer for the relationship to evolve than it otherwise might... I'm ok with this, it is what it is, but what I mean to say by pointing it out is that we truly don't know where things are going and are in no rush to figure it out. What with her new child, I doubt we'll take any major steps forward together soon. However, we have set no limits on what steps we *could* some day discuss taking.
If we decided it would make us both happy we could set a concrete date night each week plus a long weekend of vacation for just the two of us once per year, or we could get handfasted, or if/when I buy a house I could choose a place near hers, or we could work out some kind of legal contract that would allow me to take care of her child if she and her husband were to die, or I could actually move in with her and her family and be a co-primary partner to her along with her husband (I hope this never seems like a good option because it would pretty much have to mean my bf and I, who are discussing moving in together, had split up, but who knows what the future will bring). All of this would need to be ok with her husband, of course, but he and I get along very well so I don't see that as prohibitive.
I've made this very personal, I realize, and your relationship by no means needs to look anything like this in order to be healthy. That's the amazing thing about poly... the part of it that in some ways can be the most challenging to societal norms and which can be scary if you need certainty in your life... any given relationship can be allowed to develop into exactly what it wants to be with no script.
Which brings us to this question -- "How do you think I should consider approaching things differently?" I would suggest letting go of preconceptions and fears as much as you can and embracing the exciting unknown of what you and this woman could feel and could be to each other.
Why do this?
1) Because it could be amazing and bring things to your life you never imagined. For instance, if I was trying to keep myself from loving my gf "too much" or being too deeply involved in her life, I probably would never have gotten close enough to fall in love with her child, and loving a brand new person has been a unique and delightful experience.
2) Because there's no reason not to. It goes against traditional mono thinking, but it really is possible to fall in love and yet keep your head on straight and keep on loving and being committed to your other partner(s) just as much as before. Your relationship with your husband by no means needs to be in any peril over this as long as you show him you're still there with him and he trusts you. On her end, it's not bad for her to have your love and perhaps some degree of commitment even if you never can give her more. She's an adult, if she says she's fine and happy you have to trust her, and if a primary partnership ends up being what she wants and isn't something she can get from you, she can stay with you while seeking it elsewhere (she'd be limited to someone poly-friendly, who preferably doesn't hate your guts, but this is by no means impossible to find). Or she can leave if she decides this isn't right for her. There's no reason at all for you to feel guilty.
3) If you set limits now, before you have a clue what this relationship wants to become, there's every chance you'll end up butting up against them later. If in your mind it's acceptable for this to become something more than a caring friendship, even if neither of you expect it to, if it some day does go in that direction it'll be a surprise, not a disaster. Whereas if it's NOT ok for anything deeper to develop, if you realize some day that you want more than anything to do something that wouldn't cause any harm to other pieces of your life per se but that is more serious than is acceptable (giving each other special pieces of jewelry to wear as a symbol of your relationship, for example) you'll be faced with the choice of changing the boundary (in which case why was it there in the first place?), surpressing that desire (which could badly impact your relationship with her), or ending things. Why set up that situation?