Sex is one of the most important aspects of a relationship, if you do not perform the relationship will soon die, difference between a friendship and a relationship is intimacy...

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hey adonis_boy,

Again what many posters have said above was confidence.

I think what you need to do is take it less seriously. Yes it is love, however you can have fun while making love. Give her a smile while you're having foreplay. Don't worry about your erection or anything like that. I find that it helps me to stare at her face smile and just enjoy the moment.

Men are supposedly visual, but we are also human and we also do have emotions. I say giver her a smile and don't be afraid to have fun.

I didn't read the replies of anybody, so if this comes up repeated, I apologize.
Do you masturbate? If you masturbate at home, while you're not with her, it can cause serious damage to your sex life, I speak from experience.
I am 21 as well, and before this girlfriend, I haven't had an active sex life... 1 .. 2 partners at most, and it wasn't an "active" sex life, it was just like... sex two or thre times then break up.
When I first had sex with my girlfriend, I couldn't finish; part of it was my anxiety, having sex with a new girlfriend which I actually cared for AND masturbation, I had masturbated that day.
Well, I was kinda addicted to it, sometimes I preffered it more than sex, and it started affecting my sex life, decrease of sexual desire and creating mind blowing fantasies with porn strangers in my head.
When I finally let go of masturbation, things went better, and now they're perfect. I only do it infront of her, when finishing on her mouth ...
Anywaaaaaaays
If thats not your case, you could be really nervous about being with a woman, I don't know if its your first time, or one of your first times (2nd or 3rd partner) .... Lay off the porn, it creates a fake image of sex and love in your head... Hell, who doesn't want to bang strangers you met a few minutes ago? But thats a fake image of love with a partner.
If you're anxious, try doing more foreplay... kiss in bed, let her touch you, let her bring you to the edge, to the point where you NEED to fuck.
If you can't fix that, maybe you're not sexually attracted to her. I don't know how she looks, but I wouldn't get an erection of Queen Latifah were all over me.
Good luck!

Many people are right on this thread, this is often times mental. I had this issue when I was 16ish, and it sucks. However, I should also point out that it is probably not completely mental. Androgens, and more specifically testosterone play a huge role in male libido. I was over-training when a teen by working out 3-4 hours a day, which can lower test. Some guys have genetically lower test though. A couple easy solutions: If you don't lift weights, start. And don't neglect lifting fairly heavy weights with your legs. If you work out several hours a day, cut back.

Don't masturbate every day. Although it is debatable if this raises test over a long period, it has been showing that by not masturbating, your test spikes on the 6th day (and drops off to regular on the 7th). So, if you masturbate, don't finish for a few days. Combine this with moderate weight lifting and you should see an improvement.

Viagra is a short term solution. It helps you get over the initial hurdle and you might be fine after that. However be very careful as you don't want it to become a crutch. Only being able to have sex with Viagra at 21 is not a good place to be.
It could be that porn is the cause of your problem. Masturbating to porn is filling a dopamine requirement, in the same way as smoking does, and is not about your libido. Copularly impotence is a medical situation where you can get hard during masturbation with fantasy or porn but not during sex. It happens when the pleasure circuit has been rewired to higher levels of stimulation. With a natural libido the smell of a girl could get you hard. With a heavy porn and masturbation addiction you might get to a point where only your favourite lesbian domination scene does it.

I recommend reading through the website yourbrainonporn.com http://yourbrainonporn.com and checking out some of the accounts of errectile disfunction. If this is the cause of your issue the solution is a reboot which involves abstaining from porn, masturbation, and orgasm for around 8 weeks (sometimes more, sometimes less) to rewire the reward circuit back to it's pre porn state. Be aware that during this period your libido will fluctuate as your brain is healing itself. Knowledge is power.