Hey there. Welcome!
So first of all, I know this is way easier said than done, but truly, try not to think of it as a problem. I feel like guys are so hung about about ejaculating too early or too late, as if there's some perfect time to aim for, when really, it just is what it is. That's how long it takes you to come, there's nothing inherently wrong with that, ya know? So, when bringing it up with your fiancee, I wouldn't frame it as "let's have this big serious talk about my big serious problem" like it's disease or something.
I would bring it up more in the context of bringing up your sex life in general, like "hey are you happy with the way things are going right now?" and talk about what you like, what you'd like more of, rather than assuming right off the bat that this is as much of a problem for her as it is for you. Unless like, does she ever make comments directly or in a roundabout way indicating she sees it as a problem too?
Part of me wonders, when you finish and your tank drives, do you think it's because of that internalized shame and the anxiety you feel about coming "too soon"? Try not to think of it like your body is doing something wrong here, it's working exactly as it should! You're excited, and the end result of what's supposed to happen is happening! That's a win in my book. I dunno, when my guy can't hold himself off, I kinda view it as a compliment. Could you maybe approach it with some dirty talk, and flip the script? This is something good. "Oh baby you're so fucking sexy I can't help myself," "Fuck, you're gonna make me come" like, this is her superpower and you can't help but be captivated by her spell.
Anyway, like I said, try not to approach it as a problem when you talk. It is what it is, and it might need to be something you work around if she's dissatisfied with your sex life, but I guess what I'm saying is first figure out whether she's dissatisfied first, and then if so what.
Where during your play does she get her orgasm? If she just wants to stop once you finish, does that mean she often doesn't orgasm? I'd ask her if she wants to orgasm, and if so start there. If it's not happening and she wants it, that's the real problem, IMHO. I don't want to make assumptions, it's just you say when you try to go for oral she just wants you to hurry up and go to PIV, but then after you finish she's disinterested. What happens next?
I'd approach this conversation as, let's find new and different ways to get each other off. Explore each other's bodies, each other's erogenous zones. You spent your 20s together, but surely there's still more to learn, more to love, about each other, no? How do you feel about toys? Vibrators? Dildos? Would you consider looking into tantric sex? Contrary to popular belief, it's not about lasting forever. You don't need a superboner that doesn't quit. The whole session itself lasts a while, because you spend so much time just creating an atmosphere of relaxation, calm, and safety, and then take turns getting each other off manually before getting to penis-in-vagina action. If you're at all curious, check out this blog by one of SO30's moderators: http://moderntantra.blogspot.com/2013/09/welcome-to-extraordinary-passion.html
Also, check out www.omgyes.com if you haven't already. Even if she orgasms, it's nice to find other ways to orgasm. Just another skill in your repertoire, another tool in your box (or her box, I should say, ha!), another way to make her happy.
I'm kinda rambling at this point and gotta go to dinner. Hope this was remotely helpful! Sorry if I was totally off base. If I was on track, I take full credit. If I was off base, blame it on Friday even pregaming LOL j/k
Cheers!