Found out my husband has had a mistress for over 5 years?

After doing alot of digging after my friend revealed to me that my husband had someone else on the side he finally couldn't get out of admitting to me that he has had a mistress for well over 5 years. I am in shock wondering how I couldn't have known.I thought he was the best husband and father and I was so fortunate. How could he come home after being with her and not act any differently? He tells me he does love her but he loves his family more...what does that mean? I have confronted her and she has admitted to being in love with him too. She is also married.She is absolutely gorgeous which makes me even more sick and insecure. I have always felt attractive and secure. I have a good job and try to be the best wife I can be. I am so angry and disgusted. I have no idea what to do...If there are other people out there who have been in this situation I need to hear your story...Please any advice. I am so confused right now. My life is a complete lie.