Confused and down always. Shud i do?

At times I have high bouts of energy, i feel so energetic, so active but after sometime some kind of thoughts swipe me off my feet and i start feeling bad , low, a fear starts working inside me, i can'trust anyone, i find negative in everything, i feel nothing good can happen to me. This generally triggers due to some incident but even after it ends it continues, if nothing is there still i feel sommething bad will happen and it happens. I feel low for days, i dont feel like eating but eat just for the sake of it, i dont feel like working but still had to work, i dont want to resume my normal life but had to, i just want to sit for hours gazing and doing nothing and cry and cry! I i dont feel like living at times it is distupting my normallife. am impatient.  I dlnt want to spoil my life like this. Pl, help.what shud i do? I can't get out of it, am in immense pain. Am too paranoid of evrdthing be it my work or health or relationship. If something goes in my head it doesn'tcomes out .