I've been there. I've doubted whether or not I've truly wanted to be poly. I don't think that means you're not 'meant' for poly, or that you are... I think it's human nature to question. It's also human nature to want to run away from bad feelings.
The thing is... for me....
Poly is made up of two core 'learning' experiences:
1) Learning to be the active partner (and all the emotional nurturing, balance, etc that goes with that)
2) Learning to be the inactive partner (and all the jealousy, insecurity and self-development that involves)
Just because your fiance has dealt with you being with others, doesn't mean you're automatically equipped to have the shoes on your feet, honey
Don't beat yourself up. Of course you feel hypocritical. But that's not your fault.
Even if your fiance was (or seemed) to be able to deal with your poly activities 'better' than you are coping with his... it doesn't matter... everyone has their own rate and their own feelings. It doesn't make you a failure.
I realised something a couple of weeks ago.
I seem to experience jealousy and pangs more than my girlfriend. I'm also much more empathetic than my girlfriend. I realised that it's not that I'm "lagging behind on the poly train" - I just think more deeply, feel more strongly. That can be a good thing in poly too, on the flip side.
So don't feel bad about your feelings. You haven't had a chance to learn how to deal with them yet. And the fact that you're pressuring yourself (naturally, because you're trying to be good and fair) is probably making it even harder for yourself.
Truly, I felt the same way that you do until very recently, in terms of fear of abandonment and replacement. So that's about a full year and a bit of worrying about that stuff.
When things are new, they are as scary as hell. You don't know what box they fall into, so they seem terrifying. Every time my GF dated a new guy, I'd have that internal panic "she's falling in love with him, she's going to leave me, or get bored of me".
I struggled with this for, like I say, over a year. Incidentally, she never fully slept with anyone else during that time.
Like you, I felt hypocritical. I'd slept with another person and she seemed to deal with that so well. I'd often feel like I was dragging behind on the Great Poly Learning Train and that I just wasn't suited to it. Then, recently, I found out that my GF struggled a hell of a lot more than she made out... she just hid it. We have no way of knowing what goes on in our partner's mind, besides what they tell us or what we can see. Even if they are open and tell us all their emotions... how can we quantify that? How can you be sure that what you're feeling isn't exactly the same way that your fiance felt, just perceived differently?
How do I know if poly is right for me? Why am I okay with myself doing it and him not?
You could start thinking, make a list even, of what you like about poly and what you would miss if you went back to mono. Also, what you would love about being mono and what you don't like about poly.
If the things you don't like about poly relate to insecurities, can you work on them? Would you actually be able to avoid them if you were mono?
It's interesting that you said you were struggling to deal with the emotions and balance of having multiple partners.
We all have a different polysaturation point and this can be hit either by quantity of people... or can be hit by the types of people you're involved with.
You could date three casual men outside of your fiance and find a great balance... or you could date one extra who needed a lot of time and support and feel completely overwhelmed.
In terms of your fiance and your friend... I would never advise going against your gut feeling... but I've got to be honest...
My GF finally slept with someone recently. She had been waiting over a year to sleep with someone else. Once she'd met him, she hurried things along so that she could sleep with him quickly. I was so scared, feeling complete dread, really quite upset, because it was like this big monster that was finally about to come and kill me.
The day after, I felt really strange. I wasn't upset any more... but I felt strange about it. I didn't feel jealous as such, or even insecure... I was just left with this ... "gross"... kind of feeling that she'd been with someone else.
Then it passed. She's still dating him and I'm suddenly ok about everything.
For me, just ripping the plaster off really helped. I wasn't sure I could cope with her sleeping with another person, but I can. I couldn't know until it happened... and I wonder if it could be the same for you?
It could be that you really would prefer monogamy. Or it could be that you will feel ok about this once it starts happening. Or... there could be a middle ground that makes you happy, such as a triad, or a V, with you as the hinge.