Frightening thoughts - fear losing control - please help!

hello, I'm a 23 year old male. For many years I have occasionally felt what i would describe as a "creepy dark cloud" over me. This feeling typically comes on only in the later evening hours before bed. I just don't feel "right" at such times, although I am able to function normally, just as if i were simply in a 'bad mood'. It is a very foreboding feeling. Back to normal when i wake up in the morning. A few months ago i had a bad day, and experienced some sort of an "attack" in which i felt like i was losing my sanity; slipping from my grip of reality briefly. I felt like i needed to speak with someone immediately - someone to ground me. I felt like i was losing control over myself. In recent months since then, I've begun to struggle with intrusive thoughts which often involve the worst things i can imagine. The idea will occur to me to kill someone I love, or a friend, or myself. I am *extremely* troubled by these dark thoughts and feelings. I feel so guilty, like a monster. I try to redirect myself, but sometimes that is of little comfort...These thoughts seem to be the product of my own mind - I've never had any auditory or visual hallucinations that I am aware of. No dreams of murderous acts or anything like that, either. i feel like whatever is wrong with me preys on my greatest fears. These thoughts represent the antithesis of what I *want to be*. I don't think I am capable of ever hurting anyone, but I am very disturbed by these intrusive ideas. I hesitate to call them compulsions, but there is an element of that, I think. in recent weeks, i have improved a bit for no apparent reason. I've finally started to feel "normal" again on most days. Please, any thoughts or educated guesses you could offer would be really appreciated. I'm so scared that I've begun to slip into some psychotic zone here. What all might be wrong with me?