Trust me, we all felt that way with our first loves. Nowadays, I THANK GOD I had the ability to date around.....it opened up my eyes, and taught me a lot about real love.
Let me explain things to you from a 19 year old female's perspective, since Ive been one in the past. She is young and has many opportunities and choices laid out before her. She has educational, travel, employment, and relationship possibilities. Which does she choose? It's hard to know at this stage in life, but a five month relationship is not usually a priority above the other three. She may love you very much, which is why she keeps on coming back to you, but she is letting go slowly in her mind to pursue the things she thinks are of greater importance. You freaking out and acting like she is the center of your world is going to get annoying to her. She will feel bad for hurting you, but at the same time, your raw desperation is going to reek of weakness and insecurity, and become a turn off.
Your best bet, for yourself, is to let her do whatever it is she feels she needs to do with her life - that's real love. Holding her back is selfish. Have some self confidence. Maybe your relationship can survive this, but it definitely won't if you don't man up, and also let her make the choices she needs to. Do you want her to end up missing out on something because of you, and become bitter with you over it?