First I'd suggest figuring out if that kind of 'consensual forced sex' is something you need. Is it one of your kinks or was it one of your ex-girlfriend's kinks that you enjoyed?
Either way you probably should talk to her about it. In any relationship but especially bdsm influenced ones(one partner is vanilla and the other is into kink and wants to introduce it) communication and honesty with yourself and the other person(s) is important.
If the forced sex play isn't something you need. Then sit down and talk to her about your ex-girlfriend. Make sure you calmly explain that it was something she wanted, that you were ok with it, and because your relationship had lasted so long you grew accustomed to it. Also, make sure to make it clear that it isn't something you need, that your not asking her to do those things with you, but that you have some sexual habits that might pop up and you don't want her freaked out by it. I'd suggest introducing a safeword just in case. Explain it properly, that shes to use it if you do something she can't stand or doesn't like at all and that when she does you'll stop immediately and talk about it. That way you can 'retrain' yourself.
If forced sex play is something you need. Then maybe try introducing it incrimentally. Start of with some small vanilla-ish aspect of it. Maybe in the middle of sex you can wrap your arms around her and pin her arms to her side and just fuck her hard from behind. Keep and eye on her though, if its something she doesn't seem to like back off immediately. After trying it out cuddle and talk about it afterwards. Ask if it was alright, explain its something you wanted to try, and that if she didn't like it that you won't do it again. If she thought it was fine or even liked it ask if it'd be alright if you tried some things next time. Make sure she knows you'd always immediately stop if she didn't really like it.
