. . . when I'm lying there and I feel like I've had what I want and am ready for him to come, but he just keeps going and telling me "I'm not going to come yet because I want you to come more", a switch flips off in my head, and my mind wanders somewhere else.
. . . He doesn't seem capable of grasping that if he would NOT hold back on his own orgasm forever, I would be able to stay more engaged until he finishes... But as it is, after a while I start to feel like I'm trapped there until he gets off, and that makes me antsy, cranky, and then I pretty much dissociate from whatever my body's doing because I'm not getting anything out of it.
It sounds to me like you need to find a different way to speak up. I know that in this thread, you've said you can't even bring up certain topics, but then several posts later, you relayed to us that you sat your partners down and spoke up for yourself quite well.
So, to me, I think you might want to reframe what happens at the point when that switch gets flipped. Instead of letting the past take over the present and thinking it's all so futile, where you're letting your mind wander and basically waiting for it all to be over, can you do something different? If he says "I'm not going to come yet..." instead of giving in, can you find a way to tell him, "I don't want you to keep going," either in words or actions? Do you think you could firmly place your hand on his chest and say very clearly, "No. I'm ready - I want you to come now" ? And if he continues when you don't want him to, can you find a way to stop him, like pushing him off of you, wriggling out from under him, pinching his arm really hard to make him notice that you don't want to keep going, things like that? I think it would benefit you to really assert yourself here.
I highlighted where you said you feel trapped also because I seem to remember that your husband only fucks you missionary style - am I remembering that correctly? To me, if this scenario were happening and I was underneath someone who insisted on doing his thing without respecting my request, I would feel trapped, too!
Do you think you could shift yourself out from under him and find a different position? For me, because I have lower back issues, I am constantly changing positions. All I do is say, "Let's do it this way," and move into another place, onto my knees, sideways, on top, whatever. I have to, or my back gets really achy. If my partner doesn't want to do it that way, he'll either do it in the position I want for a little while before moving me into another position, or he'll come up with his own idea and say, "How about this!?" I don't recall having many partners who only stay in one position.
Now, I am speaking as someone who hasn't had the kinds of past traumas you have experienced, so I can't entirely relate to what you go through, but please don't think I am being glib and saying "just do this or that!" I understand that it's very hard for you, but I did want to acknowledge that you have spoken up when you needed to and you have been able to very eloquently and clearly say what you needed to say to both your partners, but I don't think you give yourself enough credit when you do.