Hi Amy,
I am dealing with a similar situation. My husband is also 45 and he left me out of the blue last April. At first he claimed there was no one else. Then he said he had a crush on someone and that must mean he no longer had feelings for me. Then I found out he was in a full on sexual affair with someone 20+ years younger. You can see my name here is "cliched." My husband is a doctor. His affair is with a first year nurse. He also recently told me he wants a divorce and I am finally at the point where I am beginning to understand that this is not my fault, that I will be OK without him, that this whole thing is ridiculous. My guess is that your husband is having AT LEAST an emotional affair with this woman, if not more, and he is bring cruel for making you feel like it is anything you did. I had a ruptured disc and knee problems and my stbexh got very active in martial arts and later said that my physical problems slowed him down. Nice, right? This is not normal behavior. This is not what spouses should do. This is not what love it. Don't torture yourself thinking about what you could have done differently. I am so sorry you are going through this. It is truly awful.