Couple of years - this was five years ago now. It took a while to get over it, but I am about to have a baby with my absolutely adored partner, and I am really happy. And he feels ALL MINE (I know that's a rather crude response...)
I got into the relationship with the dad with child after the breakup of a previous long relationship and you know what? I think I had lost track of the sense that when things are right, you feel peaceful and calm, and you actually look forward to the future. Starting a life filled with anxiety about the future is hardly ideal.
The other thing is when a relationship/situation is right, you don't think about it or turn it round in your mind much, you just live it! If you're thinking about it constantly that is just so exhausting. I bet when you were fantasising about meeting The One as a teenager it didn't involve you feeling crap and crying all the time.
I think I also felt that I was dealing with the fallout of somebody else's marriage and babies when I hadn't even had my own! It just felt really unfair. I'd worked really hard to get my life sorted and I think I realised I just wanted to be offered something similar.
I really do sympathise with your situation so much, but all I'd say is that you sound like a sorted person on your own terms, and it is OK to want someone to offer you similar and yes it's a horrible conflict but you only have one life.
Have you thought about taking a break from the situation for a couple of months so you can really think about it? Or seeing a counsellor on your own ? (which is what I did and was brilliant).
Quite honestly, it is entirely in your bloke's and potential stepdaughter's interest that you only take this on if you are really going be able to keep embracing the challenge.