I wouldn't even bother with the question about whether your behavior indicates lack of love for your husband. I see two separate, related problems:
* Your dissatisfaction with your marriage
* Your pain and fear about the consequences of your relationship with Jack
I would recommend the following course of action:
1. Break it off with Jack so you can concentrate on rebuilding your life, with less stress
2. Throw yourself into fixing your marriage. Try
www.divorcebusting.com. It has a lot of things that you, or you and your H, can do without a therapist.
3. Give it 6-8 months and evaluate where you are. If you don't see big changes that make you feel very hopeful about the future, try something totally different.
Therapists are definitely not a panacea. They can actually cause more problems than they solve.
Good luck!