There is only one thing you can do encapsulated in this is several actions which lead to one hopeful outcome your emotional freedom and freedom from guilt

It seems to me that your mother in a way is treating you passive aggressively the way he treats her she is obvsioulsy in a very unhealthy relationship but does not know how to and does not want to get out of it I say does not want to because she turns to you for company or friendship when he is not there but will not exclude him when he is

Now is time to tell her that soon you will be leaving for college and you will simply not be there for her to turn to so you wish now to spend some time with her one on one before you leave If she refuses you have the option to tell her that you have done all you can to try to keep the mother/daughter relationship going but her boyfriend has got in the way

In all fairness you cannot be expected to carry the guilt of your mothers choices with you for the rest of your life Yes it is time to move on and when the boyfriend is gone you may have the chance to rebuild with your mother

Hoping and wishing for you a positive resolution