Good to see you are getting help, OP.
One other smaller thing I think you can do is to try to lower your expectations on yourself as a parent. It is easy to get carried away and believe that our children have to respond to our commands as if we'd pressed a button; otherwise, we've failed as parents and they will probably grow up into thugs.
But the truth is that most 18mo aren't really developmentally ready for that kind of interaction. The trick is to manage your day in a way that doesn't set them - and you- up for failure. So instead of commanding them to do something, or stop doing it- tell them what they are going to do and then help them carry it out. Be hands-on, but in a firm and gentle way.
-Stroke pussy nicely, whilst holding their hand and guiding it. (and be prepared to pull the hand back to save poor pussy)
-Now we're going to say bye-bye to the park, while picking them up and putting them in the buggy. (if they scream, you haven't failed, it's just normal and does not reflect on you as a parent)
-Let's help you get dressed, while guiding their arms into the coat. (If they struggle you haven't failed, etc...)
And when all else fails, simply carry them off in a fireman's lift.
There will then be a probably rather longer period when they understand what you want from them perfectly well and also understand that they are expected to obey, but are testing the limits of their own autonomy. Some children do this more than others. Here screaming might have some kind of effect, but it's also likely to trigger more rebellion (as well as making both your lives less happy), so best kept to a minimum.
Again, a rebellious 2yo or 3yo who flat out refuses to do as they're told and lies on the floor of the supermarket kicking and screaming is not a reflection on your parenting: it's a stage. But again, things you can do to make it more bearable.