Resentful with money

I’m a stepmom and my husband has a daughter who is medically fragile from a previous relationship (circumstance). The daughter, let’s call her Char, has nursing care 24/7 (circumstance). Recently her doctors and the school have decided that She cannot go to school full time anymore because she frequently becomes ill. (Circumstance). My husband and I may need to contribute a certain amount of money each month now to help her get to and from her half days off school and would split this with the ex wife (circumstance). The thing is, I find myself getting really hung up on all of the money we are currently spending and sending to the ex and the kids. It amounts to about 75% of my husband’s paycheck. We both have good jobs and have enough to cover our needs but barely. We have very little flexibility when it comes to additional, unexpected expenses. I am noticing that I feel resentful of the fact that now we’ll have to send more money to his daughter. I know this comes with the territory of step parenting. Our financial situation is tight right now but our income will go up substantially in a few years given my husband’s career path. I expect what I contribute to go up as well. I used to think that when we have more money, I won’t get upset about spending money on the kids, but I’m not so sure that’s the case. I feel like our life is very constrained by the money we send to the ex and the kids and I need help setting the forest through the trees. I realize that I am the one causing myself to have these negative thoughts but they’re so deeply ingrained in really struggling to see things differently. What can I do to change my thoughts here?