She spoke to a therapist yesterday and is speaking to him again next week...Today she has been absolutely awful to be around
It may be that the therapy session has begun to shake a few things loose for her, she feels terrible and it's coming out.
If the therapy works - and it's very hard to go through sometimes, hard to stick with it- then in time things will become better, but in the short term there might be a lot of difficult behaviour. She might have to want to stick with therapy for herself though, not for either you or her dad.
It might help if her dad could talk to her about his spousification and change to a more normal interaction.
About the lack of love when she was small. This does have an effect, yes, but the stability of you being there will also have had an effect - your simple physical presence and some caregiving.
They say now that brainwiring goes on for many years, even lifelong, though naturally the first twenty are the most significant. But this means that your daughter, from age 6, did get love, and that will have sunk in to a very deep level.
If her father can come to a healthier way of interacting with her that will help a lot.
best of luck tickerty. Your love for her and worry shine through.