ooh, i wanted to add something... if it is depression, since you're an introvert, perhaps you could try building up your own inner world. hmm... how do i explain that? for me, things that bring me happiness are very much solitude types of activities where i am building up my own inner world of beauty and wonder and learning and growing, etc. sometimes it's hard to muster up the energy to do things like paint, or clean my room, or read a book, but often if i can just force myself to do it, i notice how much happier i feel after i start. but then again, i'm coming from a completely different mindset and point of view.
however, i am a big believer that in order to reach mental equilibrium, one needs to be balanced internally and externally. my problem is, i get too focused internally (Fi/Ni looping). so i pull myself out of it by finding beauty in Se types of things to help offset this. but same if i'm focused too externally, i lose my inner realm and start to feel a loss in who i am. and then there's also, sort of stumbling through the world, not focusing internally or externally. find your own personal starting point (and strangely for me, this usually involves cleaning my room, as this helps equate out to clearing my head) and try working from there.