How do I explain to him?

How do I explain to my boyfriend that I am not getting what I need from the relationship. I cannot pinpoint exactly why I am feeling this way. I think he can be very selfish at times, and very unappreciative of all that I do for him and our relationship. I have let him treat me very poorly in the past and have taken him back and given him more than one chance. since then he has not repeated the past problems, but i still feel our relationship is lacking. I do not feel appreciated, and i occasionaly feel unloved by him. But I cannot pinpoint these feelings exactly. please any advice.. i want to talk to him about this. i want to tell him that i am unsatisfied and unhappy with our relationship.