No, this isn’t love. Love doesn’t hurt like this. Healthy love brings out the best in us. It doesn’t control a person or prevent a person from enjoying life.
I can’t make a diagnosis on the basis of a letter, of course. I would need more information than you have provided here. However, one guess is that you have some variation of an anxiety disorder. Anxiety is not at all uncommon for a young teen. There is much to be anxious about in the social world. Many young people are plagued with doubts about how they look, act, and feel. Sometimes those doubts get so intense that a true anxiety disorder is born.
In your case, I wonder if you have channeled all that anxiety into concerns about this one person. If so, it is serving the same function as a phobia. Phobias narrow the anxiety into a very particular situation or person or animal, etc. In a way, it frees the person to feel okay as long as the object of the phobia isn’t there. For example: A person with anxiety might develop a phobia about small dogs. As long as he stays away from small dogs, he can function fine.
Please consider seeing a mental health counselor. A counselor can hear your whole story and can ask the questions that I can’t. Even an initial session will give you some answers and some suggestions for how to rein in this obsession.
I wish you well.