I think a lot of relationships can become a habit to the extent that it's easy to lose your own sense of identity. Perhaps the questions you should be asking yourself are:
Do I feel my life wouldn't be worth living without him? and
Do we still have the same things in common which we first had and are we still as compatible?.
If you weren't that bothered about the thought of him cheating on you, perhaps it's a subconscious wish that is giving you an excuse to break up a relationshyip which no longer has the same meaning as it used to, in which case it's probably time to summon up the courage to move on, or at least have a trial separation to sort out your true feelings.