Hi, I'm an 18y.o. straight guy. I can quite easily make myself cum when masturbating, but when it comes to sex I really struggle. My current gf is my first sexual partner. During foreplay and sex, I get hard and I feel aroused but it doesn't last very long - even when she's masturbating me, or giving me a blowjob, I seem to feel less hard than if I'm just 'going solo'. It seems to be worse during penetrative sex, as I just partially lose my erection within a matter of minutes. I have no problem making her cum, but I just can't cum from her. I can masturbate with her, but it seems to be more effort and more difficult than if I'm just by myself. Is there anyway to fix this? I'm worried she may feel that I'm not attracted to her/she can't please me, which certainly isn't true! Thanks.