You're NOT a bad mum!! And you're not alone.
I joined mn 2 months ago because I was having the exact same problem with my 8 yr old ds.
From everything I've read on here, it is just a phase they seem to go through at this age, although a very very trying and upsetting phase.
We tried everything with ds1, grounding, bans, taking away toys. The only thing that has worked so far, as strange as it may sound, is making him help me with housework.
When he starts a tantrum, I get him to start cleaning with me. It uses his energy, gives him some time to think, and I'm not pushing him away from me so he finds it easier to talk to me.
I think when we sent him to his room, he was screaming more through fustration. It's like having an 8 year old teenager, and I think they find it hard to explain how they feel.
Obviously every child is different, and I do think 'bad' behaviour needs to be dealt with, but giving our son the chance to explain why he fells like this and what he thinks would help has worked wonders.
Having said that, there are still times I could scream at him, but it's alot better.