Based on the information you have supplied, I have no doubt that your wife is cheating on you. I would definitely confront both of them, but separately. I would talk to your best friend first and be very calm about it. Put on an oscar worthy appearance. One of the biggest indicators of cheating is a decline in sex and affection. That's because she most likely is getting it from someone else.
This is not your fault. I don't care if you gained weight or haven't been mr perfect. 9 out of 10 times, the cheater will rationalize that its not their fault, but it most certainly is and in this case your best friend is not a friend at all. If it turns out they have a sexual relationship, cut the both of them out of your life. If they don't have a sexual relationship, it's time to put your boot down and set boundaries. Best friend or not, he should never be play wrestling and drinking one on one with your wife. That's wrong and they both should know that.