Need Serious Help, Somebody Please!?

Okay to cut a long story short my Mum and Dad broke up which I took quite well up until now, My Dad had been seeing anther women behind my Mum's back for months and pretending to go somewhere but really visit her. Once again to cut a long story short my Mum gave my Dad a second chance and we've been staying all together for 2 weeks now until I found a letter from the women my Dad had been seeing. I decided to throw it away to stop any commotion between my parents and to set another argument of. He suffers with diabetes and severe clinical depression which doesn't help the situation. I suffer with severe lower back pains, stress and anxiety which again doesn't help again with the situation. Anyways back to this letter I found, I decided to see who it was from and what a guess it was from the women Dad had been seeing, so I decided to throw it away only for my Dad to see it had gone missing in the next 2 days. So basically he's had another huge argument with my Mum and he's threatened to throw something away of mine, all I have is my computer and phone which cost much more than a stupid card. I'm now worried as hell and need some information on what to do next, the situation is very sensitive so please reply positively.