Its illegal for her to have a relationship ie a new boyfriend whilst married to you. If she is committing adultery whilst married to you, you may be able to have both your daughters, as you say she's unstable. If you can prove this and want to take it further action id recommend a lawyer.
If I was in your situation I would also take a police officer with you. They will go with you as a witness and also incase she does decide to go crazy you have your back covered. You'll have to pay him though, maybe 1,000 to 2,000 THB. A Thai on your side is good support against another Thai.
Be careful when alone with her, or avoid it as my ex-wife tried to stab me before. I was alone and she had 2 friends with her and she made up some story that I hit her and her friends backed it up. After she come at me with a kitchen knife I started to video her and when she noticed I was videoing her she fell to the floor and started some BS drama.
At this point I called my landlady and she said she was on the way to come and see us both. While I was waiting for landlady to come I called the police and they said that they'd already had a call about my address, so the landlady had called them for me.
This was all in Chiang Mai. Within 2 days id moved out and moved back to Pattaya and started again.
Hope it all works out for you.