As-salāmu ʿalaykum,
I wanted to ask about the validity of a divorce according to Shīʿa (Jaʿfarī) fiqh.
My husband said words of divorce to me, but there were no two male witnesses present at the time. He had looked through my phone and seen messages to another male. I got defensive and then he said Inti talaq. 

Since then he has been staying at his parents, he told me I need to move out. I have a daughter and he has a son to a previous marriage. He refuses to let the children see each other this week. 

I have sincerely repented to allah swt and i vow not to seek outside distractions and focus on our marriage. I have also made it clear to him how sorry I am and taken full responsibility for my actions.
Could you please clarify whether this divorce would be considered valid, and what steps I should take next? Has my iddah period started now or does it not start until the divorce is done in front of witnesses.
Jazākum Allahu khayran.