@Shefliesonherownwings others have explained it to you but OP also tried to leverage the fact that she did more than her ex previously in the decade or so before he changed his work schedule to have the DC on a 50/50 basis.
She has become entrenched in believing that he owes her from these previous years prior to 50/50, and took this action to try to compensate herself historically.
The mistake she made was the time difference technically, as by the time she tried to claim from him, he already had the kids 50/50.
She continued to pursue it believing he owed her money from the last 10 years and this was how she would recover it. This is why everyone told her not to do it, because it was out of date by this time, and that he would win based on what their situation is now.
She didn’t listen
It transpired he had been paying her £1000pm maintenance the entire time anyway, and when it went to 50/50 he was still paying it - just less than before at £300pm.
So she went after him for MORE money as she felt the reduction was too much. Following chasing him for more CMS, he applied for and won 1 x child benefits for their child leaving her with 1 (1 each). This angered her even more so she continued to pursue him and now here we are