You are speaking of joint physical custody, joint legal custody, or both?

It may well be that your ex's mental disorder creates a change in circumstances sufficient to convince a court to revisit the case. However, I suggest that you try to find a lawyer to help you - it is not easy to handle this type of proceeding yourself. (You also may benefit from or require expert testimony in relation to the underlying disorder and how it affects your ex's parenting ability.) You may wish to look into legal aid and other possible sources of more affordable legal help.