What Could Cause Ejaculation Dysfunction?

I have a problem with being able to ejaculate. I'm usually fine if its been a few days between the times me and my girlfriend have sex. But when we do i can never the same night or the next morning and even sometimes the next night. If i help myself out i can usually do it a few times a day. I don't understand why that is. It's not causing any problems between us but i know she sometimes feels like it's her fault. Is there anything i can do or any medication for this? I've read also that it could be something mental but i really enjoy my sex life with my partner and there's a lot of times i feel like that i can finish, and i get that sensation but nothing happens.