This is not uncommon, my first son was extremely affectionate, he is 13 now and still is. My second son, 3, was not. Extremely independant. He had been since he was born. Around 2 was a big turning point for him and I. He always came to me if he had a scrape or nick, but not a lot if I just wanted a hug. Well, I kept trying and trying and now he gives me hugs and kisses all the time. We made a game out of it to even sometimes I am like oh no please don't kiss me... and he will do his very best to smother me in kisses until we are on the floor, rolling around, giggling. He is just his own person, with his own personality, and you know that when he gives that affection, he means it 100% and to me that is not a bad thing, it is a honest reaction and fills me with joy. He is a delight and I am glad that he is that way and his brother is the way he is. They are fantastic.
Keep trying, but not overbearingly. I felt the hurt and pain since my oldest did the lovey dovey stuff, but all of that is behind us. They will come to their own understanding and that takes time. Show love with others and they will see that and begin to want that too.
I see that you are a single mom, as I am, and I am back in college finishing up too as you are. I understand that need for affection and appriciation that we look to the children for sometimes. If you have family, make sure they know what is going on and when they come to your home or you to thiers, kiss them and hug them Hi and let your child see that. Do not have the friends or family make the kids hug or kiss them,but simply say hi how are you or something. Never force the child to give affection.
Good luck and remember, your child does love you, she just has her own way to express herself.