whenever you have a load that is near the limits of what you can do always keep it "low and slow". Keep the bucked only about six inches off the ground. If it starts to go over, drop the load. It will usually right itself. Things happen really fast. Keeping it really low also helps as it doesn't have room to go over, The bucket will hit the ground before it can go over.
Glad you are ok.
Don't let your tractor lay on it's side.
Get it upright as soon as possible.
If you can't, then see if you can get the battery out of it. A battery that leaks will cause a long lasting mess. If it has leaked, wash it with liberal amounts of baking soda and water.