when I'm with my girlfriend my erection isn't is hard as it is when I masterbate

When I'm with my girlfriend and were about to do sexual things (oral etc) most of the time I don't have an erection, and when I do its not as hard as it is when I'm masturbating by myself. When I kiss her and stuff I get one sometimes, but when it comes to doing oral and things like that I don't or it isn't as strong. I'm 14, almost 15 and I don't get it. I also take a while do ejaculate. By myself, I can do it under a minute, but when im with my girlfriend I don't get to ejaculate, the only time I have is when I had to do assist. We tried to have sex but when it came to actually putting my penis in it wasn't hard. This really frustrates me, I'm worried. I'm not gay, I love my girlfriend and at times she really turns me on. So why am I not getting a full erection and why do I take ages? Why when I was going to have sex I didn't have an erection? When I'm watching porn it goes up straight away You're help would be much appreciated