Because you're unhappy. You were unsatisfied in your marriage, and you cheated. For a year. Not a one-night stand that you later regretted. You cheated on your husband for a year. I'm not going to judge you, but don't assume that your feelings of unhappiness and discontent are just going to magically disappear now that your affair is over. There's no "affair genie" that can make things go back to the way they were. The unhappiness is still there. In fact, now it's even worse.

You were all ready to divorce your husband for your lover. Now that your lover is out of the picture, you assume that these feelings will just go away. Sorry. Doesn't work that way. You wanted out of your marriage, plain and simple. Unfortunately, instead of doing the right thing and telling your husband that you want a divorce, you just cheated on him. Now you deal with the fallout: your lover gets cold feet, and you are stuck with the fallout of infidelity. Oh, your out is suddenly gone? Sorry, thems the breaks when you cheat. Sometimes things don't work out. Look at your husband-- he got cheated on. Do you think he wants to have devoted 20 years of his life to a marriage that ends infidelity?

You were using the affair as a crutch. Clearly you needed to visit a divorce attorney a year ago. Now you've hurt your husband, ruined another woman's marriage, and lost a relationship with a new guy in the process. While things are undoubtedly worse, that doesn't change the fact that you were ready to walk out the door before your affair was discovered. Even now, with the affair discovered, you still feel like you want to leave. U R UNHAPPY. I cannot make that more clear. Unhappy people shouldn't be together. Life is too short. 

Obviously after 20 years it would easier to stay in your unhappy marriage. Staying would be selfish-- you would be lying to yourself and your husband. You cheated for a year and planned to leave him. This isn't like you were planning a trip to Disneyland and all of a sudden your flight got canceled. You wanted to be out of your marriage from the moment you laid eyes on your lover. Probably before, in fact. If you love your husband, you can try to work things out through counseling. But it sounds like you stopped loving him a long time ago.

Do yourself (and your husband) a favor and go through with the divorce. It's okay, even after 20 years. At least you won't go another two decades being miserable and wishing you had left after your affair. Or worse, stay with him and then cheat again. You wanted out of your marriage and that caused you to mess up big time. Now do the right thing and close the door on this chapter of your life. Learn from this, and try not to hurt people in the future.