I can relate to much of what you shared. My husband and I have been married for 7+ years and together for about 10. He grew up in a very "traditional" family and from the beginning I was very anti poly (what's it they say about the one who protests the most?) It took a lot for me to come to him and admit what I wanted deep down. For me I didn't even realize it until Monster came into my life.
Lobster doesn't set up boundaries, says that he loves me too much to tell me no. At the same time, he sometimes says that I'm trying to change him, that what I want isn't right. He used to say that I didn't love him enough, but he's come to understand that I don't love him any less. We've been at this for over 3 months now and we still have good and bad days.
I found solace in this forum. Was able to find others who's advice and shared struggles helped. I count myself lucky in that both my guys are willing to work on things and are friends on their own (as long as the subject of me is avoided sadly) I do find myself with guilt at times, and there are times when I just want to throw in the towel and go back to the "normal" way of things but I know that I'd never be happy so we keep trucking along. When I have my little Debbie Downer moments, one of the guys normally reminds me that we've only been at this a few months and we are all doing it from thousands of miles apart (none of us are on the same continent at the moment lol) There are people here who have been working on this for far longer and they still have ups and downs. I would just say that if you truly care about your current relationship, give it time, they might surprise you. The things I worried the most about telling Lobster, where actually some of the things he had the least amount of struggle accepting.
I think knowing that you are not alone goes a long way.