OP, I'm so sorry you are going through this.
What's interesting though is that you say you have been so happy.... then in the next sentence tell us your partner is bipolar but refuses to help himself, is always up and down and you are essentially his carer..... I am sure your daughter is wonderful and brings lots of joy, but with her additional needs, on top of a partner who needs you as a carer..... I am wondering where you are in this..... who takes care of you and meets your needs and gives you your joy?
And that is before he goes off and prioritises his own desires by having an affair..... while you are caring for him and (I suspect) also doing the majority of care for your DD?
And now you are 'helping him through' his depression, due to his affair being discovered? You must be in shock right now..... but I wonder if you are so used to the caring mode and prioritising his needs, that it hasn't occurred to you that you have every right to be deeply angry? And even to end the relationship?
He had no right to do this. He is an adult, who chose totally selfishly to betray you and have a long affair. Even now, when you know, and are in immense pain, clearly with your own mental and physical health suffering to the point of hospitalisation..... he won't look after your needs before his own preferences.....
When will your capacity for empathy be exhausted? I don't know that your husband is consciously manipulative, but he is certainly taking, taking, taking, not giving anything that I can see, while betraying your trust and treating you terribly.
You deserve so so much more.