I'm sorry that you are still hurting. As an outsider - I don't think that text from your ex MIL is nasty at all. She also wants you to stop hurting.
It is good that you acknowledge and accept responsibility for what you did that caused problems in your marriage. None of us is without some fault. But your wife also had the right to decide to stay with the marriage or to leave.
And although infidelity is terrible - do you really want to rub it in her mother's face? That is not fair to her at all. She knows - but it was not her who did it - it was your ex wife.
Are you still in counselling? Sometimes we benefit from it for a while then plateau but after a time - it is good to revisit it as new things crop up. The holidays often bring out some of the emotion, regret and sorrow to us and some professional help is the right way to go.
It is time to let go. Forgive her for her wrongs just as you expected her to forgive you, yours. And accept that she has moved on.
You are moving through the stages of grief. Anger, hurt, disbelief, sorrow. Eventually you need to reach acceptance. And a counselor can help you get there.
The good news is that you are moving through the stages. Movement is better than being stuck.
Again, I'm sorry that you are hurting and I hope you can find some ways to help you focus on some of the joys of the holidays.
