after 15 years want to take ex husband to court to re negotiate terms of settlement due to him forcing me to use his lawyer when i was mentally ill

Typically, a settlement in a divorce (separation agreement) cannot be reopened after 5 years by statute.  However, where your claim is based on mental illness, the contract may be voidable depending on the circumstances.  Alternatively, you could consider attempting to modify certain provisions of a separation agreement based on current, changed circumstances.  You should consult an attorney to discuss your options.