My mental health

I'm 16, and lately I haven't feel like myself. I don't know how I feel or why am I feeling this way, because I don't see a reason why (I have a family and friends that love me, a good home and a great school) but most of the time I feel empty, or sad, or I don't know how I feel at all? When I'm with my friends that feeling gets better (I usually forget about it, and feel happy), but when I'm alone I cry for no apparent reason and feel really anxious, and I think to myself 'Why are doing this? Just stop.' It started last year, in the end of my school year (May or June), then it stoped during the summer, and this year it came back, hard. I have been having troubles sleeping too, and I have a pressure in my chest that is always there. I dont know if that is just a fase, and I dont want to worrie my parents either. (Sorry if I made any mistakes, english is not my first language)