@PinkPoetAgain1
I think the 'why' of your 'wanting sex with him all the time' is extremely complicated and the only way to truly know the truth of it is to examine it in depth with your therapist. We can give you our opinions, but we cannot give you your truth. Only you, with your therapist's help, can do that.
As far as his behaviour (hand on neck, having to be 'dominant' to finish) yes, I think it's all wrapped up in his need for control. Control isn't always 'out in the open', sometimes it's extremely subtle to the point of only really being seen by the abuser. At those times it's to reassure the abuser, not to show domination to the victim. The victim may just have a feeling of 'something's not right here' but be not be able to put their finger on it.
If you want my guess, I think possibly subconsciously your 'desire' is based on 'if I do it with him he won't do it to me'. With maybe a smidge of "If I want sex with him that means what he does to me isn't that bad. Otherwise I couldn't bear for him to touch me". But IANAT so I could be (and probably am) all kinds of wrong and my guess is worth less than the paper it might be printed on.
Keep your therapy appointment. I cannot stress how important that is.
And yes, go to your mum's. And do consider telling her more than just 'we need time apart'. You don't have to describe what he does to you in gritty detail. I'd think it would be enough to say "Mum, he sexually abuses me. Please don't ask me to go into detail it's too painful to discuss". Honestly, if you were my daughter, I think that would be all I'd need or really want to know. And even if I wanted to know more, I would respect your boundary.