Joint custody, only clean 6months!!!

Hi,
So id like to keep this as short as possible, ive never been on the wrong side of the law, however i fell in love and had a child. 4 years later i found out hes been cheating for most of our relashionship.
I have a 3 year old and 14yr old (not his).
Long story short, i caught him cheating, through so many lows, i tried to commit suicide. His lifestyle with threats to our family home etc, proved very difficult for me. I very much regret it. But i felt at the time he might change his ways by me doing so.
Unfortuntely it was short lived. His cheating has suspected me to drugs now and hes filing for joint custody?? I was at my lowest point and afraid hes going to use that against me in court.
Im shameful of it all but really dont want to lose my kids?
I suffered alot of emotional abuse, finished with the relashionship and moving forwards, but now have this hanging over me?
I really thought he had changed and promised me he wasnt cheating or doing drugs.
I did not want to die, i wanted him to feel how frightened i was, and thus move home, but it turns out he wasnt the man i fell in love with.
Any advice would be greatful.