This probably happens to a lot of guys. Whenever I’m getting really stressed about something, I can get 80% erect, but then I won’t ejaculate and will eventually get frustrated and soften up. It’s almost certainly mental, since I’m rock hard and good to go when I’m feeling positive and confident. All kinds of thing can affect this with me (positive developments at work lead to a few days of extra horniness, too much emphasis of on having sex every day to try and conceive ASAP actually kills my libido, etc).
My guess is that since you’re 19, it’s probably mental with you too, unless you are morbidly obese or on drugs. So take a little time to sort out what’s eating you in life or in the relationship. If there are big external factors at work (deceased loved one, unemployment, clinical depression, bad relationship with the girlfriend then you probably need to deal with those issues before worrying about bonking your girl. That’s just your body’s way of setting priorities for you. It would be good if you can include the girlfriend in the process of figuring things out.
If it’s something mild like boredom in the relationship and performance anxiety, then that’s easier. Spend some time together just hanging out and having fun with no expectation of sex or having to perform. (If she begs for it, and you really don’t feel like it, just tell her so. She’ll probably be cool about it if it clears up within reasonable amount of time.) Talk a lot, and try talking about new things that you’ve not discussed with each other before. Within a couple of days, you should be back to peak performance. Relationships aren’t all about sex, and in a very real way, the intimacy and bonding of the nonsexual aspects of your relationship provide fuel for the sexual bonfire. In a long term relationship, you need to replenish that fuel every now and then.