[20/m] I'm selfish in my relationship with my girlfried(18/f). How can i change for the better?

My gf and i have been dating on and off for about 5 years, this last run about one year strong, and we are very much in love. However, I make the relationship way too much about me. She feels neglected and says it doesnt feel like a relationship because of how i act and i feel like the worst human alive for it. She gets very frustrated about this, and i change, but that only lasts about a week until i am back to my usual selfish ways. how can i really change? i feel so bad and need to make this up to her in my actions.