I don’t know if you’re picking the wrong partners. I do know that you’re right: Your anxiety about sex is the problem. The fact that you can ejaculate while masturbating shows that your plumbing works. But successful masturbation doesn’t necessarily translate to loving sex with a partner. You can orgasm quickly and efficiently when you are doing self-service. Yes, you get the release. But you don’t get the intimacy.
My guess is that you are too focused on orgasm and not focused enough on your partner. Change your goal to giving and receiving closeness and pleasure. Long lazy times in bed, hugging, kissing, cuddling and giving your partner what makes her feel good is its own kind of sweetness. Put your worries about your own orgasm aside. Let your partner know it doesn’t matter if you come or don’t. What matters is that you’re together and sharing an intimate time. If you can make love generously and selflessly, you will eventually relax enough for your body to respond.
I wish you well.