It seems like you have a lot of pain and doubt, all of which is completely understandable.

While it would have been best for him to immediately disclose when he started therapy, he did start to tell the truth. Every moment is a new opportunity. Could it be that through the process of therapy he's learning that it's safe to be honest?

One thing I have learned in both professional and personal realms is that people from dysfunctional backgrounds often lie because they don't feel safe telling the truth. On the outside it seems very bizarre. As they learn new ways of being, they eventually learn that lying no longer serves a purpose for them. It's tricky for us on the other end - and if you've read my previous post(s) you'll understand my struggles with dishonesty in relationships - but I do believe in encouraging the behavior we want. If you desire (and you deserve!) more honesty, then I invite you to consider expressing some gratitude that your partner is learning to be more honest.