Misbehaving toddler

How do you handle your misbehaving toddler? I have an almost 3 year old and an almost 1.5 year old and I'm almost 16 weeks pregnant. My oldest daughter NEVER and I literally mean NEVER listens. She's very defiant and she does whatever she wants. I don't know what to do at this point and my blood pressure has been high this entire pregnancy because trying to get my oldest daughter under control stresses me out so much. I tried gentle parenting for like 6 months and it did nothing. I've tried time outs with her and then calmly talking about why she was in timeout after and it does nothing. I've tried yelling (though I hate it) and it does nothing. I know she understands me because she's extremely smart. She can spell her name, count to 30, and is just so so smart all around so I know she understands she's being naughty and she understands everything I tell her. I try my best to be patient with her and I usually do good for a while but then she goes and breaks my patience every single day after I've asked her not to do something for the 100th time and she still continues to do it. I need help. I'm losing my sanity. My husband leaves for work Monday morning and he works out of town and stays in a hotel so he doesn't come home until Friday evening. Most days are more bad than good and I don't know what the hell to do. I try so hard to enjoy my time with both of my daughters but it's hard when I'm constantly having to redirect my oldest because she's doing something she shouldn't. I love my girls more than anything but lately I've just been spending most of the day counting down the hours until they go to bed and then when they do go to bed I feel like shit because I wish it would have been a better day

Thought I would add: She gets so much attention. I play with her and her sister all day long. We play doctor, we read books, I'm working on teaching her to identify letters, she's started tracing letters, we're working on counting higher, and so much more. She gets so so much attention.