Wow, this is tough on your son.
There is probably a lot of unresolved feelings about the divorce the separation and you moving out and not taking the boys with you.
Sometimes, I think divorce can be harder on older kids than young kids.
I was a child of divorce and it is amazing how even now as an adult, certain feeling will rush over me in certain situations with my parents. and it has been 36 years since my parents divorce was final..
Your son is in college, so going to therapy with you would be very difficult for him to have to deal with, but I think it is something you 2 should discuss for the future or next summer.
His priority should be college, so, maybe during the holidays you can have a sit down and listen to what he has to say.
PLEASE do not take offense to what he is feeling or may say. If your son is like me, this is making him feel very guilty, but it may come off as selfish, childish and uncaring of YOUR feelings, but it is a defense mechanism. If you think you can handle it, let him know you are strong enough to hear his real feelings, his concerns and his needs.
You are at a different point with all of this. The main reason is because you got to make all of the decisions and your sons did not..They need to deal with this. Your son sounds like he needs time, and he is just not ready to have a new member of the family yet.
You also need to explain to them that you are a woman, with needs and how you feel about your fiance. You may need to be honest about how you met him and why you are wanting to be married. Let them know why you love this man. Let them also know he will NEVER try to replace their father and you and he would never expect that. Let them know they are free to speak about their father, and their memories of their childhood at any time even in front of your fiance, so that they will not feel like that is off limits..
Your son sees you as his mom, not as a woman in love with a man they have never met. That is completely foreign to them to now see you as a woman who is in love with anyone outside of their family is strange. Also it can bring up feelings of why could you not love our father, when we are so much like him.
Congratulations on your new life. I am sending you strength. You do deserve to be happy and to be loved. Just give them some time to come around.