Babysitting and need advice (slight rant)

I watch my two nephews, a three year old and a ten month old. I'm also 20 weeks pregnant with my first. Recently, I feel like I'm easily frustrated and quick to anger when dealing with the kids. He's in the terrible threes and before it wasn't bad but now he never listens to me, cries for his mom and throws tantrums what seems like constantly. And I just feel like I'm mean to him. I try not to be but it's day after day and nothing changes. Not like I do anything drastic, but if I do tell him to clean his toys or something of the sort and he doesn't, I put him in a booster seat which is time out or send him to his room. But neither really work or help. I'm just wondering if anyone has advice as to what I can do and if this means that I will be a mean/terrible mom.