There is absolutely nothing wrong with looking after yourself. Putting yourself first isn't the same thing as being selfish. If someone is causing you harm, the correct thing to do is to get away from them. You have every right to take whatever steps you need to take to look after your mental and physical well-being.
Why would your partner struggle to find work around your DD? Are you going to move out and leave her with him? If she went with you the chances are he'd have every other weekend, like most non resident parents. Even if you didn't plan to take DD with you, I think you need to plan for her living with you in the future, because once she's experienced life in a non toxic household that's probably where she's going to want to be.
So what if he was the breadwinner for years? You would have been contributing in other ways and that also has a value.
You're not leaving him in the lurch.
First off if he wasn't a knob you wouldn't be leaving at all, you're only leaving because you can't take care of yourself whilst in his presence. That's not just 'oh we've fallen out of love'. He's literally making you ill.
(Although falling out of love would be a valid reason to leave in itself, however hard it would be for the person left behind. It's not shitty behaviour, it's living your own life how you need to live it, which everyone has the right to do).
Second, he's not a baby and you're not responsible for him or for sorting his life out. He's capable of sorting his own life out and he will, even if DD lives with him full time. Lots of people in the same situation and they all manage. And if he can't manage it still isn't your responsibility to fix things. If he eg needs antidepressants or help from social services with parenting skills, that's stuff he can access.
You're so caught up on feeling like you have to be his rock. You don't.
Regards your DD. You speak of abandoning her, but that's not what this is. Even if you leave without her. How you describe your life, you're not really 'there' anyway. If you don't have the energy to get out of bed then you don't have the energy to be truly 'present' for her either. Being her non resident parent and explaining that you had to leave the marriage and family home for your own well-being and so you can be a better parent to her, that's not abandonment. Staying in an abusive marriage where you're a shadow of who you could be and have little emotional energy to be a proper parent with, that's far more harmful to her development.