What's your weight distribution like? Since you used to box it's probably something like 60-70% on the lead foot and 40-30% on the rear, right? Try sitting a little more on your rear leg - something like 50/50 or 60/40. You won't be able to react to punches quite as quickly, but if you're doing MMA you shouldn't be staying in the pocket all the time like you would in boxing anyways. Having your weight on your rear leg (bouncing helps too, provided you're not in the pocket) will allow you to throw kicks more easily and smoothly. You could also try a wide stance for ease of sprawling, especially if you're fighting a guy with good transitional skills.
"I don't want them to like me, because when I'm in the ring, punches fly to the face. It is not a tenderness time. I am not falling in love. And he knows that when the fight starts I am kicking faces."
- Wanderlei Silva