Can get an erection; Can't seem to ejaculate

This is my first post here so I thought I should use proper terminology. Anyways, I'm 20 years old and I have too much stamina during sex. I used to masturbate on average 1-2 times a day throughout high school. Nowadays, I average about 4 times a week.

I can cum in no time when I masturbate, but it takes some serious time for me to cum during sex. It has gotten to the point where it has affected my relationships. For example, when I don't cum, my partner feels like she isn't sexy or hot enough. This leads to further frustration for both of us.

Is there anyway I can decrease the amount of time it takes for me to cum?