What’s wrong with me??

I’m 15 and for about 6 months now I have been down. Over summer I couldn’t get out of bed or do anything. I just felt like I had no energy to do anything and I stopped eating unless my parents physically forced me too. Some days I feel like I’m ok though and I laugh and smile but I know that I am never fully happy. I just feel so guilty because I feel like I’m letting everyone I know down when literally nothing bad has happened to me. I am currently writing this in my bed on a Sunday morning and am so close to crying but I don’t know why. What’s wrong with me??