I'm sure you'll get plenty of disagreement with this but here goes: I also started with a very forward horse before finding myself with a "normal" but poorly started mare. Mostly, she wouldn't accept the bit, and it took a LONG time to get her to stretch into it.
Now, what I find is, the biggest hurdle is to get her to move freely off the leg. And to do this, what I've found is to use speed. Yes, ordinary "Go Faster M'dear!". I try to use the crop rather than ineffectual legs. Anyway, it keeps the session lively, because it's fun to go faster (even if a little out of balance- I try not to interfere when giving the Move On); there's less nagging (I'm prone to that) and when you do get a willingness to simply "go faster" you have the energy that you can work with, and can get the rounding, and coming-through more easily; which works towards collection. As Podhasky, I think, said, when in doubt, move off briskly.