Declining sexual performance. just turning 18

So I'm just turning 18 recently, and it seems in the past year or so my sexual performance went from satisfactory to terrbile. My first few times with my girlfriend, (both about 15, still with the same girl now) I could just go and go without ejaculating, and since those times it seems like now as soon I penetrate her I'm ready to blow. I can last through foreplay and oral sex pretty well, but as soon as I go in, it's like something in my brain clicks and makes me want to cum. It's the same thing when I masturbate too, as soon as there's penetration it intensifies it for me tenfold.

I'm not sure what caused this, whether it be masturbating too much (3-4 times a week) or what it could be.

Is there any way I can learn to control this? It's starting to really have a negative effect on me and it's tearing my self-esteem apart.