I feel good all day after having sex with my gf but I do not feel this good afte!

I feel good all day after having sex with my gf but I do not feel this good after pleasuring myself. Why?

Why is it that self pleasuring myself feels good in the short term yet does not last however having sex and letting it all out with my girlfriend feels good for many hours later. Why?

Because sexual acts are more than just physical release. You have to consider the mental side of sex, relationships, closeness and passion....all things you don't get when you go solo. For masturbation, you're basically firing off a round for the big ending. So it's a very basic process...like running a short marathon.
Sex offers you a closeness, some emotional happiness from the intimacy and it allows you to focus on just running the race. You're actually enjoying the trip, not just trying to simply get to the destination. Nothing can replace physical interraction when it comes to sex and desire. That ensures survival of the species. If masturbation were just as fun or good as sex, there'd be a lot less kids running around.