My dad was abused physically (not sexually) as a child and suffers with Bi-Polar. I may have mentioned it before in H&R on these forums at some point before. Growing up with him has caused some problems, such as my inability to speak to my dad about serious matters even now as an adult. I don't know who he is sometimes. He is on heavy medication (anti depressants) but his dosage has to be upped. He's in therapy at the moment but all it seems to be doing is digging up repressed memories which make it worse around the house. My mum is having a hard time dealing with it all because now I've moved out she's on her own with him.
He has two/three moods. One is normal, like any dad, cares for their family, works hard, listens etc. One is extremely infantile, sort of like he lapses into a weird sadistic childlike state. He makes ridiculous jokes and acts in quite an ignorant way. The third is the worst, which is suicidal, violent (never towards me or the family though) and depressive. More than once I've been scared he's killed himself because he will go missing for hours and hours at a time, or lock himself in a room. He does it to stop himself doing or saying things he will regret, so he explained to me.
It's certainly not fun. I would go and see a GP about it, and maybe get onto medication. Doctors nowdays will listen, my dad was ignored for years because once-upon-a-time the ultraconservative health service saw it as pathetic for a man to have such issues. It also damages my dads pride, he's a working man and has been bricklaying his whole life - he's almost expected to be a mans man. His father was an old-school Policeman and was very violent towards my dad. As a result he had a big lack of respect for authority in his 20's. He drank a lot, was well known to the Police and the community etc. If it wasn't for my mum he would never have straightened out. He often says my mum saved his life, he'd probably be in prison or dead if it weren't for her.
If you are concerned I seriously advise you see someone. Bi-Polar/Manic Depression/Depression doesn't just hurt you as a person, it hurts everyone around you. I'm perfectly willing to tell you all about it if you want!