My only suggestion is take her into an arena or some other kind of enclosed area with lots of space. Ask for the trot in a circle and if she speeds up, LIGHTLY tighten the inside rein to make the circle smaller. I don't know what kind of bit you are riding her in but this is easiest in a snaffle. It will take a while because she clearly has never been taught to collect or had specific cues installed for go, stop, turn, slow down, speed up, etc. It seems like they just kinda rode her a lot but didn't teach her much. If she tends to stop when you try to post, can you sit her trot out and work on it that way?
Another thing that works for getting a horse to move "with their brakes on" is to ask for the jog, then if they pick up too fast or start to speed up, stop them and back a couple of steps. Then continue on and repeat, and repeat, and repeat, etc. Do this consistently every time you ride. If you are riding a trail, do it the entire way about 1/2 or 1/4 mile from home. You can also incorporate this into your circles. Even after she starts to really carry herself relaxed, smooth, and slow, continue it every now and then just to keep that gait and after a while, it will be a permanent gait. This method is how I got that wonderful little jog on Dobe.
Just remember that whatever you do, it will not be an overnight fix and could take weeks or months of working every day before she gets consistent but just don't give up. It does work.