The fairest thing to do is to tell your mother what you think you know and how it makes you feel. There’s no point in yelling at her or getting into an argument with her about it. She has to deal with her relationship with your dad. How she does it will probably affect your relationship with her. That remains to be seen.
I’m concerned that thinking about your parents’ issues is a great distraction from thinking about your own. You’re only 20 and have a baby. You have social anxiety and a “boyfriend” you can’t communicate with. Your focus needs to be on growing up yourself and making a solid family for your child. Please get yourself into therapy to deal with all of this. If you’re having trouble staying in recovery, find a support group. Get some couples therapy for you and the boyfriend to see if you can be a family. And if you don’t have a job or schooling, look into local services that will help you get started.
You and your baby deserve to have a decent life. Leave your parents’ relationship to them – at least for now. You have plenty to deal with in your own life.
I wish you well.