Since the two of you are already in therapy and it has been effective, the thing to do is to talk honestly about how you are feeling in session. Your therapist has probably come to understand each of you and will have much more to offer you than I can on the basis of a 250-word letter. I don’t know if you’ve given it an honest try and your husband really isn’t doing enough or if your expectations are unrealistic. Or the problem may be something else entirely.
What I do know for sure is this: Your difficulties with your husband won’t end with a divorce. You have three young children. That means the two of you will be in each other’s lives for at least another 20 years. Yes, people can and do move on. But it takes a great deal of effort to recover from the separation and find someone new while still making divorced parenting work. Ultimately, only you can decide if the effort to transform the marriage is easier or harder than the effort to be co-parents living in separate places.
Please do let your therapist know about your struggle to stay in the marriage. Whether you decide to stay together or decide to part, your therapy can be an important factor for making either choice in a way that is healthy for everyone involved.
I wish you well.