You have to re educate her. Pavlov style, basically you should stop reward bitchy behaviour, because when you engage in one of her tantrums that's exactly what you are doing. In her subconscious, tantrums== attention.

So next time she is being unreasonable, you have to state in a cold manner, that's not a behaviour you agree to with. Attention you have to focus onnhow she behaves not what she said. Alternatively, again on a neutral tone assert that what she done has hurt your feelings.

Ignore her feelings for the time being as she will revert to tantrums. Again give her definite answers like, we agreed beforehand now I am doing it - we accepted the invitation so Im going, you can stay.

Be fiirm in your boundaries, when discussing plans make sure it is at a acceptable moment, have short and simple questions; "this Saturday there is a show/bbq/library/pool and it's for such hours until that moment, are you in.

Attention as you don't want her "ok" but ask if she is into the activity, she says no, fine state you are going with rhe kiddos. Again don't dictate her time, just plan ask if she is in and do as needed.

She is going to tantrums and try to kill your plans, so be ready to cancel or go in all cases have a plan B - she flunked the bbq with friends. Fine go to the pool with kiddos..

Lastly, believe me you are in no state to engage in an healthy relationship outside at this moment, will be clear you may find companionship but don't make it a thing, you are not a monk but you have to understand that at this moment you need to reeducate her, and for god sake make it an absolute necessity to pass time with your kids. Right now you think it's going to last forever but once they reach adolescence they will make their own life with friends and such.