The kids live with their grandparents, why do I still have to pay their mum?

I split with my wife 6 years ago after she had numerous affairs, got herself into drugs etc. The house sale went through and she admitted the affairs, we divorced. The proceeds from the sale went into our joint account, and she emptied it before i could get my share, which was thousands.

My solicitor said there was nothing i could do about it because the money had probably gone anyway with the lifestyle her and her new partner where living, so i came away with nothing from the house; no furniture or anything. I tried to get the money owed somehow leagally, but her partner took to assaulting me and vandalising my property. Police where called each time, now i have to pay a vast amount to the csa and I’m not allowed to see my children. Even though the kids live with their grandparents, I still have to pay my ex? how does that work fair?

please help if possible