Is it bad to not “stop” intrusive thoughts straight away? Usually when I get one I can recognise it and focus my attention on something else but sometimes I don’t realise it and let it keep going and I think about how I would “do” whatever it is I’m thinking about. When this happens I feel so bad that I didn’t stop it and that I want bad things to happen. Does anyone else have this problem?

It's not bad. Stopping them right away might even be a compulsion. Instead you can let yourself feel the discomfort and resist doing compulsions to make the discomfort go away. Resisting compulsions teaches you that the thought is not dangerous and does not have some secret meaning, it's simply a thought. Everyone has intrusive thoughts, but people with OCD place extra weight, focus, and meaning on them. So the best thing we can do with intrusive thoughts or feelings is...nothing! Easier said than done when you have OCD. My therapist has really helped me practice this and I'm slowly getting better!