Hi there Tiecobb16, and welcome to Scarleteen!
I'm sorry to hear that you've been experiencing this! I can see why not getting what you want over and over might make you feel a little bit frustrated or unfulfilled.
I do want to congratulate you on taking the steps that you have! Talking with him about the problem and suggesting resources for learning how to make you feel good in a low-stakes environment are the first two things that I was going to suggest.
You mentioned a few times that he tends to overthink things when you two are having sex, and you think that makes it difficult for him to keep an erection or do anything other than the same position with penetrative sex. Did he tell you that he overthinks things, or is that more like your interpretation of how he feels? What about when you feel like he isn't interested? Has he said things like that, or is that what you sense in his body language (like closing his eyes or not initiating)?
The reason I ask is that we sometimes get it into our heads that we know what our partners are thinking, because we know them really well. But in times like that (especially if we don't get any explicit sharing of thoughts/feelings), our brains can fill in the gaps with our own narratives of others, or even projections of our own insecurities, that aren't accurate.
I wonder also - has he spoken at all about whether or not he feels sexually satisfied? I know you said that you're usually the one to lead/initiate. Has he said anything about not wanting or not being able to have sex as frequently as you seem to want it?
When you've talked with him about this stuff, and voiced what's making you feel unfulfilled and/or unwanted, how does he respond? Other than going to the doctor with you, has he done anything (inside or outside of the bedroom) to address how this is affecting your relationship?
Nothing happens in contradiction to nature, only in contradiction to what we know of it. -Special Agent Dana Katherine Scully