I have learned that two people who want to live life in very different ways cannot have a successful relationship.
You want to live a life in which you devote yourself to one intimate relationship.
He wants to live a life in which he is free to participate in many intimate relationships at once.
It sounds like you are trying to talk yourself into being ok with this situation when you are really not ok, and you will never be ok.
he wants me to 'stick with it and see what happens'. maybe it will not work out with this other girl. maybe it will just be the two of us again for another couple of years before this happens again or doesn't happen at all. live for the moment and then when/if it hurts TOO MUCH, then go.
Don't look to the future and hope for something that may never happen. If you are not happy now, you will not be happy then.