Thank you for contacting me again at JustAnswer.
It can be difficult when a partner makes the changes that you wanted them to make, but it feels as if it is too late. The only way that you will be able to tell if you can make the marriage work is if you are willing to end the fling, stopping all contact, and address your feelings about the marriage. Trying to decide between a fling and a marriage is like comparing apples to oranges. They are very different since the fling is generally more of a fantasy than a real relationship that involves daily responsibilities and commitments.
Ideally, it would be important to examine why you might be willing to settle for someone who is only rarely available to you and who is in a relationship with someone else. I would encourage you to explore why you don't consider yourself worth more than this. Individual therapy could be helpful with this. Also, if you decide to end the fling, then couples therapy could be very helpful to work on improving the marriage and allow you to find ways to not feel trapped any more.
I encourage you to take the time to work on you, rather than distracting yourself with the affair. Your husband has made some changes, now the ball is in your court to see if you are willing to work on your part in the marriage.
I hope this is helpful. Please let me know if I can clarify further.