Dear Christy
When I read your story, I felt so angry for you. That your H can torment you about your weight and have 2 A's with the same person (and I guess the second times is because you were still during the after effects of the weight gain/depression thing right?), well it made my blood boil.
I truly understand how depressing it can be to have big weight gains and then struggle with loosing it, but WELL DONE YOU! You are loosing the weight and picking yourself up. Now this is quite key here, you are changing, you lost the weight picked yourself up from your depression, got a job. These are all really positive things and you should be applauded. Your H has noticed these changes because he keeps coming round and back to you. Christy, you cannot change him, but you can change yourself.
Now, as to your M. Well, you have to decide whether or not you want to save it. Your post talks about both - help to get this man out of your head and then help to save your M. Only you can decide that. Have you read all the information on this site? You should do that and also try and get the book Surviving an Affair - this will help you understand better why this situation has happened.
I am appalled that you were the one to move out with your 2 children - this seems completely wrong to me.
When you know what you really want to do, and I presume it is to save your M, you will need to formulate a plan and stick to it. This is based around Plan A (about becoming the best person you can be) and Plan B (when you no longer feel able to cope as your love for your H is running away and you need to protect what love is left and yourself).
I'm not very good doing links to other posts, but if you can find a poster "Red Hat", he has some links at the bottom of his signature including things about quick starts for those who have been betrayed.
Christy, finally I am so very sorry that you find yourself here and the turmoil you are in, but you are in a good place. More people will come along soon who will be able to help you - I don't always feel well placed to give advice.
Wishing you well from London, and keep your chin up!
Lisa