I struggle with this too, I am 25 weeks pregnant.
This is some of my little tricks, although they are rarely 100% successful all day and there is BOUND to be a meltdown.
I avoid take DS to the grocery store if I can. If I do take him, it's for short trips where I know I could leave if things got dicey. I get the "car" cart and that works for abut 10 minutes. I also bring snacks.
For getting in the car/getting pants on for bed etc, I have tried letting him make decisions. I'll say, if you climb into your carseat, you can get your own vitamins (he gets vitamins in the car every morning and I know he likes getting them out of the case) or I will let you have a Squeezy. I also say if he gets in the car like a good boy I will sing with him. Either way, he's getting in the car. Today he freaked and wanted to "stay home mom NOW!!" and so I kinda forced him into the backseat of the car ( I was getting pretty heated too) and then closed the door. After a few minutes, he decided that it was better to get in the seat and have mommy in the car, than be in there alone. Last night, he didn't want to put on his diaper or jammies, so I said, you have 3 minutes to get your diaper/jammies on, if you do not put them on, I will not be reading stories to you tonight. He normally says "oh okay mommy" and behaves.
I also try to let him have A LOT of choices throughout his day, especially on things I know will start a fight if not handled properly. Instead of saying "let's get our shirt on" I say "which shirt do you want to wear today?" and then I might ask "do you want mommy to put it on or do you want to do it yourself".
He still says "no" a lot and misbehaves, but during those times you have to realize that this is a stage, it will not last forever, you can get through it.
You can't worry about the future, about what it will be like at 9months pregnant or with a newborn because all of the worry in the world isn't going to change it. You will get through it!!