What Shall I do ??

Hello everybody,
First of all i'd like to thank you all in advance for helping me.Scondly, let me tell you very briefly my story so you can have a preciser idea of what i'm going through.

My girlfriend and I are got together when we were 17. We met in high school. From the moment I saw her I knew she was the one (like in the movies) but I really felt that way then and I still do today. So I went ahead and asked her out, she rejected me because she already had a boyfriend but said that we could be friends, I was crushed but I didn't give up, I figured her "teen-aged" romance will not last forever and that I still had my chance. We went out a couple times as friends, it was akward the first times because I felt that she wasn't enjoying my company but it slowly changed and we had great times. After a year and a half, she broke up with her boyfriend and two months later we were together. On our first date as a couple I told her how much I loved her and my whole story. She wasn't in the same place as me but said she'd give me a try. So I took my chance and was being the perfect boyfriend and got her to fall in love with me as well.
Since then we are madly in love and have a great relationship. We shared a lot of things together (loss of virginity, first trip alone etc etc).
We are now 3 years later and she left on vacation in her family in Chile a week ago. We had a teary good-bye and we said we were gonna stay with each other forever.
Today I get a letter in the mail. Opening it all excited of course but it wasn't what i expected.
In this letter she tells me that she has doubts about us. That maybe we got together too young and that we hadn't had time to find ourselves. That she maybe could be happier with someone else, that maybe we are not meant to be and are still caught in a "fairytale romance" that hides another truth. To sum up she doesn't want to make a mistake. Either the mistake of staying with me and pass out on someone else or ,the contrary, breaking up with me and find someone else and then regret it because I was better.
I am devastated, I don't want to loose her, she's everything for me !! What shall I do ? Shall I let her decide ? Shall I make jealous to make her realize how she loves me ??
I am really lost I have never thought i'd be in that situation

Please help me... I appreciate it sincerely

Set a realistic deadline as to how long you are willing to hang around waiting for her to decide, let her know about it, and then back off entirely, and keep your contact with her extremely minimal, if at all. Give her a taste of what her life will be like without you in it.

Do not do something destructive and foolish to make her jealous. Your relationship is imperiled enough right now, don't you think?

Try to do all of your freaking out before she gets back, so that by the time you see her again, you aren't just reacting to what she said. The truth is, neil, that people have doubts like this all the time. You can choose to see this in a positive way; at least she didn't hide it from you. Try not to send any frantic messages to her, but don't go reptile cold on her, either. Let her know you got the letter and that you will talk when she gets back. Remember- stay in control. Don't flip out on her.

When she gets back, explain that you understand that this is a normal, natural thing for a person in her position to do but that you, yourself, feel sure about her and you hope she can feel sure about you too. If she suggests a "break", don't go for it. Breaks are total horseshit. You're either together or you're not. This might be the most painful thing you ever have to do, but if she's not keen on really being together, you have to let her go. You don't want a girlfriend who isn't completely sure she wants to be with you, you really don't.

Thank you very much for your answers... this really helps me a lot...