How scary! I'm glad she will be ok, that really could have ended much worse!
There's really no reason you should ever need to tie a horse long enough to reach the ground. If you are high-lining out on a trail somewhere, there is that, but that is a totally different thing. If a horse needs to eat while tied, you can hang a bucket or a hay bag, or hold the lead rope as they eat off the ground. That's one of those things where things can be fine for a while but then go wrong SO quickly, as you've now seen.
That's another thing you will learn as you go, you kind of have to look at everything you are doing with your horse, and take it all in and imagine what might go wrong, and then prevent it. It comes with experience though. Things like making sure the stall door or trailer door is swung wide open, not just partway. A horse may panic if it goes to go through something and catches their hip or if it swings back into them as they're walking by. Tying them long or too low or not using a quick release knot is the same. Someone might do it once and end up with a dead horse, others may do it for years before they ever have a problem.
It takes a while to really get to know horses and what and what not to do around them. My non horsey boyfriend came with me to a show once, and we had the drop down windows on the slant load trailer open and were offering the horses water before we left to head home. My mare had her head out her window and was slobbering water all over the place after taking a sip and my boyfriend thought it would be funny to put his hand in the bucket and flick the water drops on her face, and not being a horse person, he didn't see anything wrong with this. I managed to walk around the trailer just as he did it, and my mare threw her head up in surprise, absolutely nailing her face on the top of the trailer window. Thank god she just lost a bit of hair and wasn't cut, but man did I see RED, and my boyfriend sure got an earful about it! But having not really been around horses much, he just wasn't aware of how sensitive and how quick they can react to small things, but it just takes time to learn things like that.
Sh*t happens for sure, especially with horses, but as long as you are learning from the mistakes and keep trying and moving forward, that is the best anyone can do. Everyone makes mistakes, whether they'd like to admit it or not