There is a lot here — and you’ve displayed a great deal of courage by explaining all of this in your email. I am very glad you are working with a therapist. Having someone guide you through all of this is important.
Since the therapist thought it was a good idea to tell your mom, ask him or her if they would be willing to have a session where you invite your mom in to talk about what you are thinking and feeling — and perhaps to then talk about what happened. This is a very difficult situation you are in, but the fact that you ate still caught in the middle between having these feelings and the man still with your mother makes it particularly uncomfortable.
These things typically have many layers. To begin with, the internal conflict you are having and struggle with your mental well-being isn’t known by your mother. If the therapist is recommending and is willing. Taking care of this layer of communicating with your mom is important. I’d begin by talking to your therapist about this prospect and then take it from there. Keeping it a secret may be causing many of the difficulties or at least contributing to them.