CSA have enforced a 33% DEO

I have been divorced for a number of years and have throught the last 10 years paid the csa various deo ,last year they told me I am £11,000 in arrears and I will have to pay the max amount around £500 a month which is killing me to do Im having to do overtime to pay for it as Im single with a mortgage so its a struggle, when I asked the CSA to do a review of how Ive accrued the arrears they told me theyve based a few of my past jobs on what I was earning and sent me a copy they also said because throughout the years I didnt inform them immediately whenever I changed jobs this is what I now owe and they will have to take the max amount as it has to be collected within 2 years can this be right ?