Does this make me unfit to be a mother?

My husband and I have been TTC for 3 years. I want nothing more than to be a mother. It has been a tough journey so far, but this is all I want.

I have been babysitting for the same family for 5 years. The kids were young and cute when I started watching them. They were well behaved (for young children, that is lol) and we could have fun. Now they have gotten a little older, and they are complete assholes. I feel bad for saying that, but even their parents think so. It's completely the parents' fault, they have raised them into entitled little brats. It's to the point where I really don't like the kids anymore. All they do is play video games now and try to act like those douchey guys on YouTube. I can't even tell you how many times I say "shut the f... up" in my head. I don't want you to get the wrong idea, I'm really not this terrible person. I love children and I really actually am a loving and sweet person. It's just these kids. Being with them day in and day out has turned me into this negative, always annoyed person. I can't quit. First of all, I know they will never find anyone else to put up with their kids. Second of all, I have helped raise them for the past 5 years (obviously my role is not the one that stuck. There is no way in hell I would let my kids behave this way). Although I don't like them anymore, I do still love them.

Like right now, one of them is throwing a huge tantrum. At first he was just upset because his brother is going to a birthday party and he can't go. I tried explaining that he doesn't always get to do all the same things his brother does. That his brother was was invited and this was his friend, not his. I tried being really nice about it and just explaining to him how this works, but then he just kept acting like a brat about it, so I gave up. Then he was sitting there playing on his phone and started throwing a fit about the game. He kept losing, it was messing up, something like that. He was having a full on meltdown because of it. I simply said, then don't play it. He started yelling at me saying I'm not the one dealing with these problems. Now he is in his room crying, screaming, banging stuff around. And I am completely ignoring him.

I'm trying to tell myself that it will be different with my own children. They will be more likely to listen to me and I absolutely will not raise my children this way. But another voice is telling me that I can't do this. I don't believe that voice, but I think it might be winning.

I want to be a mother so bad, it's killing me. Does this make me unfit though?