The most important statement in your letter is that you understand that your obsession is out of control and that you want it to stop. That tells me that you are probably not suffering from psychosis but rather from a form of OCD. In your anxiety and perhaps loneliness, you have landed on this obsession and the compulsions that go with it as a coping mechanism. By fantasizing about a relationship with the celebrity, you give yourself the involvement and affection that your parents aren’t able to give.
You state that you are on medication, but you didn’t mention whether you are engaged in talk therapy. If you are not, I urge you to find a counselor you can trust and to work through the pain and disappointments of your growing up years. Fortunately, it really is never too late to have a happier life. With good treatment, you can learn how to disengage from your parents and to find other older, wiser people (real in your life adults, not fantasy figures) who can give you the support and involvement that everyone needs. You can also learn how to parent yourself constructively.
If you are already in therapy, I hope you have discussed the extent and seriousness of the obsession with your therapist. A therapist only has the information you share to work with. It might be helpful for you to take a copy of your letter and this response to your next (or first) session to advance the conversation.
You have already taken important steps toward healing by taking medication and by writing to us here at PsychCentral. I hope you will take the next steps and do the hard therapeutic work you need to do to give yourself the life you deserve.
I wish you well.