Why do I get so sad about her death?

Ok so my best friend Andi died 3–25-09 (Andi rest in peace) at the age of nineteen after having a heart attck, and hitting her head after having an athsma attack. She was in a coma for three day then they had to pull the plug on her :(
I saw her the day before she had the athsma attack and she was totally fine and happy and talking about her life long plan. She seemed totally happy, and healthy (besides the medical issues she had like diabetes, and she was allergic to alot).
Every time I hear the song Cowgirls dont cry by Brroks and Dunn, or the song Tonight by Group 1 Crew I get really sad sometimes to the point of tears (even in school). I dont know how to not get so sad when I think about her. I can look at pictures of her and not get sad but when I think about how she left earth to Heaven I get really upset and cry.
What do you think I should do bout this? Is it normal to still be grieving even after several months?. Any advice?