Dental adhesive can help, but if the fit is good, it's best to practice with them without adhesive, at home, when you're feeling safe and comfortable.
They don't get the suction that top dentures get to keep them stable, so they do seem to move around. Remember that every feeling in your mouth is amplified greatly. One tooth suddenly being higher than another after a filling is enough to make your whole mouth feel off-kilter. Tiny little cavities feel like gaping caverns to a sensitive organ like the tongue, which has loads of nerve endings. A bottom denture is going to feel like a huge alien chunk of plastic that you can't control - at first. Like all things, it needs steady practice, and your brain will adjust.
Wear it while you watch TV. Talk often, to the pets, plants, the TV, thin-air. Singing is especially good for training your mouth and tongue. You might feel hopeless. You might want to cry. It's a process! Once you're healed, don't shy away from trying to eat! You might look at a hamburger and think "I can't do this", but try anyway. Eat slowly and carefully. Your tongue will start to figure out what's going on and what it needs to do, and it will start to do it without you needing to consciously control it.