You may be the antihero in this story but I'm rooting for you. You can fix this but you have to genuinely understand that what you did was morally wrong, not just legally wrong. Neither your children nor your ex will forgive you unless you're actually sorry.
Right now you're stuck in this defensive and destructive pattern of feeling like you're owed something and it's everyone else's fault. Instead of YOU alienated your children by trying to get money you weren't owed, you say HE alienated them against you. Instead of YOU failed to listen to the evidence based and rational information provided here, CMS led you wrong. You have to take accountability.
You took £300 a month, an amicable co-parenting relationship, and 2 lots of CB and threw it on the roulette table, choosing to trust the 1-2% of posters who said you could win more, rather than the vast majority who warned you not to, and now you have to have a long think about the thought patterns that got you there and disrupt them before the destroy your chance of any relationship with your kids.
You came here with an AIBU and got told yes, YWBU, you continued. Are you ready to finally consider that maybe, we were right?