being that life isn't the easiest course to take and everyone has to deal with it's consequences, i believe that you should be honest with him. no matter which way you tell this to him, it won't be easy and he won't jump for joy at the thought of you leaving. of course it will break his heart but if this is what your decision is, to leave him, then you're going to have to face reality and tell him so.
i've been in your shoes before where i didn't want to be with a girl any longer and she did have a low self esteem. honestly in the long run, they might not change but if you point out this one fact about them, they may hate you for it, but eventually they will see a part of themself that they need to work on. life always brings happiness, but how often does it bring pain. ask yourself this. raverboy
...this is just my perspective on the situation...