It's especially hard when we have a lot on our plate... .like college! I'm so glad you're seeing a therapist.
I have felt guilty and so has my husband. Right now we are VLC with his sister and parents. I think the guilt is there because on some level, there were good times, happy moments, etc. But then the monster came out. My hubby occasionally feels guilty because he hates to do this to her, to cut off communication and hold the boundary that she HAS to be willing to work with us in the relationship (it's her dysfunctional way or the highway). She is more interested in being right and having us apologize (for the lack of contact) than having a healthy relationship with her son. And hey, that's her choice.
The guilt gets better. One thing that has helped me is recognizing the difference between guilt and false guilt. Coincidentally, most of what comes our way is false guilt. It's easier now since we've been dealing with it in healthier ways to let it roll off. It takes time.

This sounds funny, but you just get to a point where enough is enough. You sound like you're pretty close to it. My hubby got there when his sister was being triangulated by their mother and she called him a liar. Then he drew the line. I hit the point earlier where I could no longer sacrifice my integrity or mental and emotional health. I almost immediately drew several boundaries and would not give. It's simple for us because with the LC, we've now had the opportunity to have peace - to focus, like you're trying to do, on being happy, on having a real life! If anything gets in the way of our happiness, if we're feeling upset and angry, then we know something isn't right. Something needs to change and that's when we adjust our boundaries.
When we're happy and others feel we're too happy for what's 'really going on' in the family our long response is this: "I have made the choice to focus on what I CAN do and what I CAN change. I am choosing to be happy. I am choosing to enjoy my life by being positive. This does not mean I disregard the pain and issues. It simply means I will not allow the problem to swallow me whole and I recognize the solution needs to start with her. When she gets to that point, I will be more than happy to help in whatever healthy ways I can."
Boundaries are exhausting. It's terribly hard to hold the line and not give in. The only reminder for me is that without my boundaries, I'd be a mess. The last thing I want is for it to go back to the way it was. Consider it like training someone... .a few times and then they catch on. Then they realize it's not worth the effort in bringing it to you. They find other outlets and avenues for their frustration and venting.
It's a lot easier for me to be as assertive as I can, to be short and sweet and not allow myself to be baited into an argument. I grew up in a very passive/passive aggressive home and it took a little work to be assertive. "I'm feeling like my boundaries aren't being respected and I need to hang up now. Maybe we can try talking again when you're ready." "I am feeling really attacked right now. I need to remove myself from this conversation so I can think about how I feel." "Shelbypup4141, you're sweeping it all under the rug!" and response - "I'm sorry you feel that way."
There's a book by Susan Forward and Donna Frazier called Emotional Blackmail. I read it right after my hubs and I got married and it was immensely helpful in creating boundaries. If you have the time to read it, do so! It's great for helping those of us who struggle with boundaries to build those skills up in healthy, non-offensive ways. I still keep it around for a resource!
I'm glad you joined the board! It's been really helpful to me too. It's nice to know that we're not alone in all of this!