These have been predominantly due to miscommunication and his assuming my response to a situation incorrectly. I feel like I have invested huge amounts of my time & energy to fit in with him, yet he doesn't appear to reciprocate.
You've identified 2 separate issues: Communication and reciprocation. Communication is relatively easy to fix. Reciprocation is not.
I've been married for 36+ years now. My wife, at least once a week, would say something like,
"Oh, I thought you would have,..." I now recognize it as "indirect language" for (1)
"I was expecting you to do,..., but you didn't." OR (2)
"I'm disappointed in you for not,..." Of course, my typical response is (1)
"Why on Earth would you think THAT?" (2)
"That would have never entered my mind." (3)
"If you wanted me to do something for you, all you needed to do was ask and it would have been done."
How does one spell "assume"? Easy to remember.
"It makes and ASS out of U and ME."
I operate on "direct language", she operates on "indirect language". Now, over the years, she's gotten a lot better, but I have simply learned to be direct and ask those clarifying questions. However, every now and then, she simply doesn't communicate, at all, and assumes. I think she has a habit of forgetting perspective. In other words, if SHE were in my shoes, SHE would have done "that task", whatever it is. She forgets, I am not her, and I have my own thoughts, priorities, and plans for the day. Communication is everything. The vast majority of conflicts in every relationship can be narrowed down to miscommunication.
The reciprocation aspect, well, that's something that perhaps you two need to calmly discuss. As you are sensing, it really sucks when the relationship seems "out of balance". There can also be confusion around each other's "love language". Some people need to actually hear the words,
"I love you." Some people express their love through their actions, not words. If you're in that sort of situation, there needs to be some communication. If he is one that can defensively rattle off the top 20 things he is doing for the relationship, and you aren't recognizing it as his "love language", then that needs to be clarified. On the other hand, there are people that are more inclined to be "entitled" and are "users". In which case, now you've got a real problem that you can't fix.