Deployment

It's coming down the chain. A friend of his from a previous unit saw the notice go across the desk. It's set for end of December at this point. Assuming nothing changes... But it's a year. I'm not sure what I'm going to do.
He's suggested half heartedly that we get a third. A woman who will be OUR girlfriend so he's sure I'm emotionally and physically cared for while he's gone. I am bi-sexual so this has come up before. It's been a year and a half we've lived together. Almost 3 years of dating and he's admitted he's planning a proposal around September once he's bought the house and we've settled in. We're committed.
But I don't want someone else. And I've said that, but he's worried I might cheat while he's gone, because I cheated on a long distance boyfriend before. I'm 25 and that ex was when I was 17. 
I don't know how to reassure him, to show him, that I'm strong enough to wait. To get through this possible year for him, for us.