Trapped and cracking

Ok guys I really need some advice. I have absolutely no idea where to start with this so I'm just gunna dive in. Here goes;

About a month ago I went into my garage one night to find my dad stood on milk crates, throwing ratchet straps over the steel beam in the ceiling. His intention was to hang himself. Now before you say cry for help, he had zero way of knowing I would catch him, it was sheer luck I was still up at that time so I know he's serious. I managed to talk him down and get him inside. Once he was inside I hid every house key and sat by the back door on guard all night. He says it was a minor blip, a lapse of judgement. I know this to be a lie, I can see it as I've been in that situation before. My mum knows about the incident and assures me not to worry but we both work full time and have to leave him alone ion the house all day. I can't not worry and it's driving me mad and even affecting my work. I just can't concentrate and my bosses are losing their patience. This recent rock in my foundations is spiraling my mindset downward too and I fear I'm slipping back into depression but who can I turn to in this situation? How can I tell my mum that not only is her husband on the brink but her first born son is losing his shit too. I can't talk to my dad about it for obvious reasons, he said he felt we would be better off without him so if he gets even the slightest idea that he had a hand in sending me to a bad place...well it doesn't even bare thinking about. I honestly feel that the only thing holding my family together is me getting up on a morning and faking a smile all day while I'm literally cracking inside. I'm scared to take any time off work to try and gain some perspective since I'm already on thin ice. going to see someone would probably raise alarms with my parents too. Something has to be done, after a month of holding this in I finally cracked last night and flat footed one of my best friends in the face because he had the rotten luck of sitting on one of the earlier mentioned milk crates and went to touch my dads motorbike. I can feel myself turning sour and I'm not sure I can do anything about it. My world is falling apart around me and I don't know what to do. 5 other people watched me fucking Leonidous my mate and now none of them will talk to me either.

Help??