Hey
Grief comes in stages. The days leading up to my grandmother passing were horrible (about 4 days). When she actually did pass in the hospital, I was an absolute wreck. I cried, wanted to scream, and just ran out of the hospital room. I couldn't deal. However, when it was time for her funeral a few days later, I was just numb. I just sat there, emotionless, stating at her casket. Then I walked up to her casket, placed a picture of myst in there, kissed her head, and sad back down. Then when we were going to the cemetery, it's like I was on auto pilot. I didn't cry, I didn't feel any emotions. I was totally numb to everything...
A few weeks go by, and I turn into an absolute wreck. I can't sleep, I can't eat, and my stomach was in knots. It finally hit me....and it hit me
hard. I never lost anyone in death before. She's still the first person I lost- I was 19 at the time.
Even after 4 years since her death, I have my "bad days". Days where I feel like I just want to crawl up in a ball and cry because I miss her so fucking much. She was like my best friend. I wish I could tell you that it's easy, but it's not. Coping with death is fucking hard.... At least for me..:
If I were to get in her old car, I'm sure that I'd feel the same way you did with your friend. There are so many memories in there. It's where you laughed, listened to music together, probably sang together, went places together... It's hard because there are so many memories. I know this will sound cliche, but it's true- your friend doesn't want you to be so sad over this. She wants you to be okay. There's no brining her back, but just think of it this way; what advice would she give to you about her passing? Maybe something along the lines of: "don't cry for me. I love you. I want you to be happy! Enjoy riding in my car too, it's okay! Just make sure no one hits it"
So I guess the bottom line is- what you are feeling is normal. It's part of the Grief process. In time, you will get better and it won't hurt as much when you think of it or are surrounded by things that remind you of her. I couldn't even look at my grandmas rings before without breaking down and crying but now I wear them everyday.
Here's an interesting link I found for you:
http://m.helpguide.org/articles/grief-loss/grief-loss (sorry that it's mobile version, I'm using my phone)
Also, one final thing! Have you thought of making something in remembrance of her? I made something I named a "memory box". It's about the size of a shoebox, and I decorated it to customize it. Then inside of it I put a vial of her perfume, pictures, and other miscellaneous items that were hers/reminded me of her. When i really miss her, I open it and it brings me comfort. You have to find what works for you, but you will get through it
I have been working on a memory type thing, I gathered up some old pictures and pinned them to a cork board in my room. Once a week I go to the cemetery with a friend or my sister, it's nice to just sit there by her headstone and think of good times. Earlier today I met up with those same friends and we just sat around telling stories about our time with her, It was nice reminiscing. 
