Matt, it sounds like you could use some counseling. I just placed my dad (97) in assisted living after caring for him in my home for the last 4 1/2 years…and yes, there’s a significant amount of guilt involved. My feelings towards him, my inner monologue towards him were getting increasingly toxic and several months ago I got into counseling to deal with that reality. My counselor helped me sort through guilt, resentment, and the events that triggered those feelings; helped me face up to the difficult decision to move him out of a living situation that was working pretty well for him and not at all for me. One of the most useful concepts she has given me is that emotion is information. The emotions generated through the endless slog of care giving are real and legitimate, each and every one of them, and they are giving you clues about why they keep coming up.
For me, much of the resentment comes from our contrasting roles through our lives, his emotional absence from mine as I was growing up and my need to be present for him during his decline. Each incident of decreasing competence on his part and corresponding feeling for me that I needed to help out, to fix, to be sensitive, just triggered resentment over his lack of concern for my struggles and needs as a child. And each time I felt resentment, that triggered a feeling of guilt, Vicious cycle.
Your feelings are clues, they point towards possible unresolved issues, unhealed wounds. In reality you are a Rock Star for caring for your Mom and her significant other, so many “kids” just turn their backs. Feel those feelings, examine them. They are real, they are legitimate, and they are informative.