First of all, I just wanted to thank ALL of you who replied! Even those who didn't really have advice but just let me know I wasn't alone. It's been super refreshing and helpful to read all of your replies!
I'm kinda pressed for time at the moment, but you better believe I'll be coming back and reading this thread some more and really asking myself all the questions you guys posed and really taking some time to think.
One thing I did want to address was to you nycindie... I must have come off wrong. Both him and her have not pressured me AT ALL. And they have (very slowly) been developing their relationship without me involved. They have both been very clear they don't want to hurt me and are okay to be slow. Multiple times they've both told me they are okay being friends if I can't handle it. But I can tell they both wanna be with each other, and I really don't wanna hold either of them back! When I said I was kinda jerking them around, I didn't mean about my involvement. I meant that some days I'd tell them I didn't want to be poly at all (meaning they'd have to end their relationship as more than friends).
Plus I have always felt it would be easier for me to deal with my fiance dating another woman if I was dating her as well. That way I wouldn't feel as left out and I could be more certain of her feelings for me as well. Once I voiced this opinion to them both, they were both into it! So it was actually MY doing bringing up her and I getting involved. (Of course this isn't the only reason I wanna be with her, I find her very attractive, she's one of my best friends, and I really like her for her... I enjoy spending time with her!)
In any event, the three of us FINALLY got together yesterday and had a long discussion about it (at this point it's been my fiance and I talking, her and him talking, and her and I talking... but no three way convo). I think it went really well... We got a lot out and talked for a long time. The three of us spent the whole day together, went on a movie date, and then my fiance even gave both her and I massages while we watched another movie in our bedroom. It was all really comfortable and fun.
I was having a particularly bad day when I made the initial post! Yesterday has really given me hope... and both yesterday and today I didn't/don't feel any jealousy at all. I know that it's not gonna be easy and I'll still have jealous moments... But realizing that they both really care about me and it's not about some sort of malicious intent or "stealing" anything from me really helps.
I think most of my problems with poly are just related to my own insecurities, which I can and want to work on. Doesn't mean it will be easy... But I believe anything worthwhile takes some work.
I really appreciate you all re-iterating that I shouldn't feel bad because I'm not quite as good as him at being the inactive partner (thanks a lot for that sparklepop). It's nice to hear. I just hated feeling like a terrible person because I had so much trouble with it. Neither one of them has ever made me feel that way! It was me. But I was always of the mind that I was being a hypocrite and un-loving somehow since I had such a hard time with it.
Anyway... Thanks again. Live and learn, right?