Infidelity Support Group
Any relationship in which one partner engages willfully in sexual relations with another outside of the partnership is considered to have experienced infidelity. This breach of trust is often traumatizing for the faithful partner as well as the relationship, and support is often needed to heal emotionally and to decide whether or not the relationship should continue after...

Realize regardless of how you end the relationship that your new relationship will not end well as neither of you can be trusted.
You know that he is willing to go after a female who is already in a relationship and he knows you are willing to cheat on a significant other when the going gets tough. As such, each of you will always keep each other at an emotional arm's length away.
If you don't do some serious soul searching and change who you are then every relationship you are in until you do commit/admit to change will fail.
You are growing, you are changing, you want different things, you want to experience new things.... and so tell him that. Be gentle to him, but tell him you are very sorry but you feel that you are growing apart from him, and it's not his fault, and that you need to end this relationship.
And as forwardmotion said, take some time..... to just date some different people, take the time to decide who and what you really want to be with in life, and do not jump into another relationship right away.
Always think if the roles were reversed.
Can you put yourself in that mind space of TOTAL devastation?
Growing up sucks but eventually you gotta