Just sad and don’t know why..

I’m not sure why I feel so sad all the time but I do. I am 16 and I feel I have a good life, yet I feel sad I don’t know what to do I’ve felt this way for around 5-7 months where I will do something incredibly fun and amazing and walk away feeling nothing at all, but I should. I also get this feeling of complete loneliness and sadness. Any suggestions on what I should do?