My son was diagnosed at twelve too, he is 15 now. He used to have the most appalling violent meltdowns but that has completely stopped now.
This was in part thanks to camhs doing some CBT using a traffic light system where he was encouraged to stop, think and do. Also some work with him on anxiety which was the driving factor behind them.
I also read the explosive child and stopped punishing him for his meltdowns, recognising that as he was overwhelmed with remorse after because he already knew it was wrong, but at that point in time could not stop himself. Instead we worked in him being able to recognise on a scale of one to ten how anxious he was feeling and also worked on ways for him to self soothe and eventually take responsibility for that.
We have other problems now, and although his meltdowns stopped his anxiety actually worsened and he is on medication. But the meltdowns were horrendous so I feel that his other issues, although bad, are not as stressful to deal with.
Does your local authority have an autism team? I attended a course run by them which was very helpful, and they also did some one to one work with both me and my son.
I would also say 'pick your battles' is an even more apt phrase with an autistic child. So ds has to go to camhs appointments without fail but does not always have to tidy up his room if he has had a stressful day.
And you are not out of your depth. You have been parenting a child with autism for twelve years already. It's hard but it is a bit easier with a diagnosis. I find people take me more seriously when I say DS can't cope with things.
And don't be afraid to ask for adjustments. I had to ask if there was a spare room we could wait in in A and E once because DS was really struggling in the waiting room. The staff couldn't have been nicer and I think it's good to show DS that it's ok to say I can't cope with this, I need an adjustment to be able to do it. It's what he's going to have to do for the rest of his life.