I'm sure you're not awful. Your thinking is distorted, understandably since you are suffering from depression and PTSD. Hypothetically though, and I do not think this is the case, if you were 'awful' you would still find understanding and forgiveness from many people. Firstly really awful people are not traumatised and troubled by thinking they are awful, they don't care. They think they're great, they're just in it for themselves. Other people may do something bad (and for all I know you didn't, but are blaming yourself), and then feel troubled by it....they are usually met with understanding. You might be surprised at the human capacity for redemption and recovery. You seem to me a good person who has been through something traumatic and has turned the anger inside.
I also think that whatever you fear from telling people would not be as bad as what you are going through now. You feel trapped right now because you fear telling anyone, yet you know that could help. Just thinking that may make the fear worse, but I suspect the minute it is out that fear will crumble away. I think deep down you hope you'll find the courage. Maybe the first person won't understand, but many would, so don't give up. There is no pressure to do anything right now this minute, so don't veer between fear about telling someone, and being angry with yourself because you can't.
We've all got regrets. We've all done wrong and done things we wish we could change. We've all felt guilt over things that weren't our fault too.
I think you are set in a very rigid pattern of thinking - that is not a criticism, we all are - and to some extent it may have helped you cope. It might be worth questioning some of it though - when you tell yourself you are bad or don't deserve help, just pause and think, 'What if that weren't the case? What could another perspective be if I had to come up with one?'. Also when you get advice, don't always think that it must mean you're doing wrong or that people are annoyed, question that too and think, 'that could be it, but if I assume for a minute that they value me and want to help, how would I interpret their comment then?'. I know you might say you can't make that assumption, but you are making assumptions if you assume the opposite too. It is far more likely that people will wish you well.