It sounds like you had a causal relationship with someone and developed deeper feelings for her than what she had for you. You are probably more hurt over what you were hoping things would eventually be between the two of you, than what they actually were. This girl might not even know how you really feel, because it was never communicated. She’s probably just doing what most people do by keeping her options open until she gets a boyfriend.
Nothing is wrong with feeling disappointed, that’s a part of being human, but you should take time to self-evaluate to understand how a person who was giving you so little in a relationship can have such a big impact on your self-esteem.
If you don’t identify why you may be sensitive about getting attention and affection, you may come off as being too needy. That will scare her and other girls away.
If you still want to pursue this girl, confirm whether or not she has a boyfriend, get to know each other as friends and then discuss what type of relationship you are interested in having with her. Also, keep your options open until you are in a committed relationship. A guy your age should be socializing and getting to know different girls, before settling down with just one.