Why can't i let myself be happy for once?

ok, so i have had my fair share of bad things, at one point i cryed every night, now i only cry on some and instead of feeling sad all the time i only feel sad once or twice a day (first time in about 5 years) but i still cry soemtimes for no reason, sometimes if someone says something minor to me (like your fat) which before wouldn't have bothered me but now makes me run out crying, but apart from that i am happy (well i feel really happy because i have been sad for so long, its getting better) (sometiems i am just randomly sad) and i still have some problems but i am learning to just get threw it, so why do i still feel so sad, i tryed lying to myself i was happy but it din't work, and i am happer than i have been in 5 years, so why do i sometimes feel really down, like crying and curling up in a ball and never waking up as i said ive still got some problems, all i want is to be happy ... why can't i just let myself be happy for once ps im only 15