I agreed to pay £300 a month but now I need to reduce it

I have been split up from my partner since the end of May. Despite getting only £1200 a month I agreed to pay £300 a month towards my son directly to my ex. Now I have come to a point where I need to reduce it to the recommended amount as given by the CSA calculator.

My ex has made things really awkward since I have met someone else and starting 09.08.13, has refused me seeing my son, whom I had for 24 hours each weekend and saw for 2 hours 2 nights a week. She contacted the CSA and they wrote to me requesting me to set up payments from 13.08.13, for £170 per month. Even though I have paid £300 for August already that is not taken into account, neither is the fact that she refuses to let me use any of the equipment or toys or anything that I paid for back when we were together.

Even though I have over paid by £390 in the last 3 months I have to make another payment of £170 for August making this months tally up to £470. Almost half my months wages. Obviously the £170 would be less if I still got to see my son when I wanted to and how I have been every week for the last 3 months and how I intended to going forward. Again, the fact that she is refusing access out of spite because I have met someone new isnt taken into consideration.