Sweets, if you know he's cheating based on what the woman said (and you believe her), and your boyfriend won't talk about it...there's no point in forgiving anything. He's not asking for your forgiveness. He's asking you to look the other way while he cheats at will.
I'm sorry, but this is tearing you up and he doesn't seem to care, and is trying to blame you. Not a match made in heaven.