First off, you need to forget about this high school sweetheart business. If you had a happy home situation, would you even be tempted by this other guy? Probably not, which is why you need to see this attraction for what it is: a symptom, an escapist fantasy. You have a home and family situation tht needs your attention. Taking care of current business needs to be your 100% priority, and daydreaming about the past will only prevent you from handling it with grace and with your head held high, which is what your kids need from you. Period.
Second, you said your husband used to be good with the kids. So what changed? Did he get a new job, or a promotion, demotion, different work responsibilities? Have there been lots of layoffs at his work that have him concerned about the future or taking on too much more? If he has had a drastic change in his behavior, you need to sort out what changed. Talk to him. Tell him that you are concerned that he is under a tremendous amount of pressure and it is affecting your home life and you want to sort out what has changed and how you can handle it together. Try counseling. If he won't go, you should go by yourself to help you sort stuff out.
And, for your children's sake, if you leave your husband, don't run into the arms of someone else. You need time to stand on your own and sort your own life out. Jumping into a relationship with someone else will only put your children lower on your priority list whether you want to see it that way or not.
Sorry if this is harsh, it is a huge pet peeve of mine when people allow fantasies to interfere with their real life.