To an extent, I think your gf is correct. I think your ex is taking a lot of the things you do for granted, and if you need help and she doesn’t show the same willingness to do what she can, it’s not fair to YOU. I don’t see it as putting your ex above your gf, I see it as your ex taking advantage of your kindness while not doing anything to help you when you need it, and I think that she needs to either embrace the team aspect of being parents or learn a little more independence. 

I can’t fault you for wanting to help her, because kindness is an excellent thing to teach to kids. It’s especially beneficial for them to see examples of exes treating each other with respect. But you should take a step back sometimes and let your ex fix her own problems because your priority should truly be your gf and the home you have to offer your kids.