21M and 21F difficulty in having good sex

Regarding sex life.

When I engage in sexual activity with my girlfriend. I do the foreplay really well, she's satisfied with it. But when it comes to intercourse I don't even last 5 to 10 seconds, I thurst for 4 or 5 times and then I just cum. It really upsets me not able to enjoy sex and not satisfying her too, but she's soo lovely she won't mind it she rather consoles me that it's ok. I don't know what to do.

I want to add a few points about myself.

- I don't watch porn it's been around 6 months, since my relationship.

- I don't masturbate I engage in sexual activity only with my girlfriend, and the frequency of such activities is always around 10 to 20 days

If someone's wondering the reason why I don't masturbate is I feel it's like energy wasted on something done alone which just serves limited selfish gratification. I would rather prefer spend it with a person I love which builds the relationship stronger. It's just a idea of channeling energy for a greater purpose. Some might think this as thoughtful or stupid but thats how I go about it.

I was wondering if it's happening because I stopped masturbating or I seriouly don't know. I don't know what the hell to do. So please help me out.