First rule: don't fib about how much you enjoyed it and don't fake it. If someone thinks they are hitting all the right buttons because you aren't being truthful about how many they're missing then they'll carry on doing it that way and you won't get the satisfaction you deserve.

Secondly, never have sex out of obligation. If you don't want it, you don't have to do it. Forcing yourself to have sex just to please someone else/keep them around only reinforces any negative associations you already have.

Third your ex is a selfish, lazy tool. His penis is not a magic wand that can make any woman orgasm just by pointing it in her direction. Work on dropping that idea from your mindset because it's created a mental roadblock that you can't get past without help. The truth is that many women have trouble reaching orgasm from penetration alone and need extra mental, emotional and physical stimulation to get their own personal best out of sex. You are not defective, you are not unusual and you are not alone.

Finally knowing how to please yourself and being confident in expressing it to an open and understanding partner will help. Reading Come As You Are by Dr Emily Nagoski might be a helpful starting point for you to begin your explorations. Good luck!