ECT 1 - Behavior Management Help!

Hi!
I am 4 weeks into my first ECT year and I am really struggling with behavior management and it is really getting me down, and I feel like I am failing as a teacher and not living up to potential.
For context - I had a really good ITT year and there were teething problems with my behavior management to begin with, but I really feIt I was on top of it by the end and could really show off my 'teaching'. I was told I was way beyond my years and it was like I had been teaching for years.
Yet I have come to my first employer school and I am really struggling and finding it so much harder than my PGCE. It is very difficult to get through lessons because I am constantly having to get attention back to the front. Students often starting shouting across the class, throwing things, make noises etc. and very little work gets done - every time there is an opportunity it feels like the students grab on to it. I have spoken to my mentor, my head of department, followed the behavior policy and even observed other teachers; I have then tried to do what they recommend and wait for silence, re establish expectations, be strict with the policy etc and it just feels like it is getting worse.
I just want to preface it by saying that everyone has been really supportive, but I can't help feel like I am failing. It is incredible frustrating and every lesson feels like its the first time I have stepped in front of a class of 30 teens before.
Any suggestions?