What you said about being worried that her symptoms are being caused by wanting it so bad? The mind is powerful and can do powerful things. If you want a baby, then I think you two need to relax and take things as they come. It sometimes takes awhile. Stop overthinking things.
We really want to have a baby together. We love each other and will be getting married in a few years. We already feel like we're in a way, married. We both think marriage isn't a legal piece of paper, it's the feelings between the couple. We've got ways to pay for the baby and we've got plans for everything.
A woman has a 25% chance of conceiving every month. It usually take a healthy couple an average of a year to conceive.
Symptoms are not a good sign of pregnancy as a lot of things (physically and mentally) can cause them.
I don't mean to offend, but is there a reason you want to have a baby at such a young age? I know age has absolutely nothing to do with parenting ability, (there are many older parents that shouldn't have children) but you have so many things to do and experience ahead of both of you. Have you thought about college, money, ect? Babies are not cheap or easy. Your life is no longer your own. Everything is about them and their needs 24/7. I'm not trying to judge you or your girlfriend, only you know if you are ready, but just things to think about. :)