I’m sure this is all very stressful. What you do is separate yourself from your parents’ relationship with each other and other people. This really isn’t your concern. Your mother shouldn’t be pulling you into it when things go badly. She was right to tell you “no” when you asked for information about her private life. You shouldn’t be playing detective by looking at her phone or playing counselor when she “comes running”.
Somehow your family has let the important boundary between parent and child get blurred. Get out of the middle. Don’t put yourself there. Don’t let either parent pull you in. You can’t fix whatever is going on with the two of them.
Do remember that each parent can be a good parent to you even if they can’t find a way to be faithful and loving with each other. You may need to remind them now and then that you need them to be parents and not to treat you as a peer.
I have a guess that you have plenty to deal with in your own life. One of the important tasks of the teen years is to work on figuring out how to be in a love relationship. The best thing that can come out of having seen your parents’ mistakes is an understanding of how you want to be in a relationship and what you want and deserve from a partner.
I wish you well.