it's almost like I don't really know him and I'm not really sure what it is that is causing him to be like that
It could be that he is holding on to something... or it could be that two people can only be SOOOO close, kwim? I mean, no matter how close you are to someone, there will be places you cannot go in them. Y'all are just now correcting the SSM situation, and now you have the cancer to deal with. It will take time to sort things out.
This is where what I call the "Assumption of Good Will" comes in. This means that, all things being equal, when you don't understand something about your partner, the default condition is that you attribute GOOD and POSITIVE motives to your partner's behavior.
BTW, I could NEVER get my bf to agree to this. Everything is zero-based with him. Start at the beginning each time.
When I asked if you ever felt shut out or like an outsider before, I meant before you married. In childhood, at school, etc.