How to satisfy my gf in bed..?

So my gf is two years older then me and I'm her first boyfriend that got her in bed and I think she isn't satified with me :/... I'm always trying my best and I want her to be relaxed and everything but she is constantly unconsciously telling me she isn't relaxed,she needs more time, and that next time it will be better and so on.. I mean it's not such a big problem but I feel really shitty and disapointed in myself when she tells me that,she doesn't notice that but it really hurts me.. I really love her and I want her to be happiest girl in the world but I just keep failing.. Please,any advice or something :/