Hey, sorry that your sitch has entered the phase where you felt inclined to move over here. I guess we all made the same move from other forums on the site to this one... Based on that, and especially my own experience in a sitch very similar to yours, you'll probably still "waffle" just a bit, so be prepared, it is normal.
At this point in time, with a serial cheater like you have, it is important to really step outside of the sitch and take an objective look at what you have. If you were a 3rd person looking in on your situation, what would YOU advise yourself to do? If you can pull that off, it could give you a bit of strength in whatever direction you truly decide to follow.
As for his excuses and any thoughts that you could have done something differently to prevent things from reaching this point, take them for what you KNOW they are. Complete bull! Sure, everyone could have done things "better" but when you are dealing with a serial cheater, someone with such an innate set of character flaws, the onus lies squarely in their court. Short of lowering your standards for what is right and wrong, or otherwise “enabling” his destructive behavior, there is nothing you could have done. And whatever you do, don’t allow this to hit you in the self esteem department. Again, it isn’t that you weren’t enough for him, weren’t good enough, etc. In truth, his behavior might have been caused by the fact that you were “too much” or “too good” for him.
So, take heart in that. Right now just try to step outside and see things for what they truly are. Then, you can choose your path wisely.