London is right..... his saying you was to busy was very wrong. He needs to take the whole blame for his action and you need to realize that the affair had nothing to do with you, that can be hard even if he says it, which is normal, but it had nothing to do with you!
No excuse for what he did will actually be a good enough reason, but it might help you decide if you want to try and forgive him if you see that he really wants to work out the marriage. I am glad that he is taking meds that is a first step to show you he is trying, but still has many more steps to go before your marriage will be back on track. First he needs to take the full blame, ask for forgivness, and then be very willing to do whatever to gain your trust and forgivness again, realizing that it wont come right away.