Practice without a rod, I know at first this may sound silly but pantomime the motion into your arm. Mimic the casting stroke with both arms at same time. You can use a rod butt or section for this exercise.
You can also try casting for real with both arms at the same time, try to get your left mimicing the right.
I switched to leftie for a few months after a hand op and found it quite easy to pick up. I could cast a decent line after not too long practice. The hardest thing isn't casting - it's striking into a fish. You'll be amazed at how slow the reaction from brain to left hand to strike is
You need to get familiar with the rod in your left hand, so take it out in the garden and side cast, overhead cast, roll cast etc. Get used to the feeling in your left hand and arm. At the same time practice the pantomime movements and you'll pick it up pretty fast
Hope that helps.
PS - If you have tailing loop problems, start rotating your wrist further at the end of the forward stroke, open your loop up. Not a good or recommended long term fix but if you find it compromises your fishing, especially with the left hand do that

Don't make a habit of it though!