Replying to Terrain Park Beginner-in need of advice

This is my first time posting, so please tell me if there are any specific posting rules I am not following.
Also, I know this should go in the Trick Tips section, but it has little to no activity.

So, to this post. I've been skiing for 8 years now, and only two years ago did I start on little side jumps. This year I tried park jumps and boxes for the first time, and I can go off of them perfectly without any problem. I can do all the boxes, but only straight forward (not grinding, spins on/off, etc.).
I've started messing around with 180's the past two weeks, and I can land one off of a small two-foot jump! I know, not much, but hey a lot more than the start of this season.
I've tried doing 90's onto a thin box, and man am I terrible at it. I always slip out from under myself about 5 feet in and can't seem to get it.

After that entire story of my park career so far; I come to my question. What are ways I can improve all of this? It seems everybody at my mountain does tricks perfectly and never falls, and I just don't get it haha. Any useful tips for getting a 180 down on a larger jump, attempting to move up to a 360, learning to grind, and essentially any tricks/tips/exercises that can help me advance from where I am right now in the park?