Your Ex still doesn't want to be a parent. He is only seeking 50/50 custody to get out of child support, as you said.
So, he is still, not thinking of his daughter. Just himself.
And your 3 year old reminds you of, your Ex. So probably that is why you are distant from her. Because, your Ex decided that he didn't want to be a Dad or a Husband, as soon as your 3 year old was born. Hence, she just reminds you of something that caused all the problems you have.
Plus you see everyone else having fun, except you.
But your 5 year old, had nothing to do with your Ex or your divorce... because she is from a previous relationship.
But, all the things you do not like about your Ex, you are displacing onto your 3 year old. So you are estranged emotionally, from her. Because, SHE is the child, that you had with your Ex, and she is the child, that maybe indirectly, you hold for being responsible for your divorce.
Children can feel the vibes of their Mom, even if they cannot articulate it.
I know someone that grew up with a Mom that resented her, for whatever reason. The Mom was cold to her all the time. And she grew up, "hating" her Mom. And they were never close. And all her life, this woman tried to impress her Mom or to get love from her Mom, but she just got a cold shoulder. Its sad... things like this can really negatively affect an individual even when they are adults. And it can affect even their own personal relationships. Because, there is no closure.
So, deal with this within yourself and get Therapy to help you if you need it.
Because, if not, it will affect your daughter, and then it will affect both of you your entire life. It will not be loving. And a child needs that, from their Mom.
And, you already seem to resent and dislike, your 3 year old. She just reminds you of, him. And you favor... your 5 year old.
Your daughter is 3. She didn't ruin your marriage. Your Ex did.... he wanted to be single, again. And act like a child himself.
Your Ex... ruined the marriage. You really better know that.
Your 3 year old, is an innocent by product of a very selfish immature man. Your Ex.
But you are blaming your 3 year old for your current life.....
But sure, you have no one to count on, not even your family is willing to babysit for you. So, find someone you trust, to help you when you need to. Isn't there someone, a neighbor or friend you can ask for help? Or call your relatives or family to come and visit you.