Just curious. Imagine the scenario where you know deep down that your DP is only with you because it's easier to stay with you than it is to spend time alone looking for someone better. You're an easier option than going through all the dating and searching again. You're an easy option when it comes to company as without you, your DP has none and so he relies on you for his social life including cinema, drinks, live music and days out. You're a guaranteed supply of sex.
You also know deep down that if someone "better" did present themselves to your DP, he wouldn't think twice. You also know that if you were to finish with him today, he would be registered on every dating site going by tonight.
You know all this, at least you think you do. You have no proof - just that odd, random niggle that comes into your head every now and again. The niggle that appears when said partner rarely contacts you for a few days after a weekend or the niggle that pops up when you remember he came back to you after the girl at work turned him down.
You have no other problems and on the surface, everything is great. But that niggle just won't go away.