Not going to judge you but what happened happened, can't turn back time. You have a couple of routes:

A) Break off the affair, find new job. Help your spouse get back in shape by going to exercises with her and buy diet package for her (and even go on diet with her). She sacrifice her body shape for your kids. - Save your family life but difficulty level = HELL mode.

B) Break off the affair and resign from your job but outsource your sex life. Just make sure you don't get caught during your outsourcing. However, this option will not help her get a satisfactory sex life unless you can satisfy her with your tongue and fingers. Again, you can salvage your family life this way and difficulty level = Medium.

C) Divorce and pay alimony and elope with your mistress. Pretty much scorch earth tactic. You'll be accused of being heartless etc but then who cares? Your life is YOURS! Just take note your new mistress might lose her figure someday and you might end up losing both family and your woman. Not the best choice but it IS a choice.

There are other choices and each with its own pros and cons (i.e. make your wife accept your mistress and have threesome etc) but you need to think what is it that you seek in life?