You are the person your daughter is taking out all of her frustration on.
Frustration of being a young single mom
being dependent on her parents for support
trying to finish school
trying to figure out what to do with the rest of her life
maybe feeling like less of parent than she wants to be
If this is true then you have two choices - continue to be her punching bag and letting her take her negative feelings out on you - or not. You seem like a great Mom - I have read your advice to others - and you are obviously greatly concerned with your family's well being. But you don't deserve to be treated this way.
You could step back and let your daughter take a more active, hands on role in the day to the day care of her child. Or you could gently explain to her that while you are more than willing to help her out until she becomes a responsible parent and adult you will not allow her to treat you with anything less than respect anymore.
If you are providing the primary physical care of you grandchild and are the one who he is with every day of course that baby is going to come crying to you first. If she wants to be first in his life, then she needs to spend more time with him, or, at least, be more present in his life so he understands that she can also meet his needs.
It sounds like she is still growing up, and still has the attitude that you should "fix" everything for her. It may be time to let her know that she needs to "fix" her own life. Her frustration, and bad behavior towards you could be a result of her knowing that she needs to grow up and take more responsibility but being "mad" at you for having to do so.
Whatever you do, know that you are doing it all out of love and don't let anyone tell you otherwise. But you have to be treated poorly - you don't deserve it.
Good Luck
God Bless