Stick with it, no dog is too old to learn new tricks.Approach the instructor before the class, and say you're having difficulty with the whole mirror image thing and if they were any good they'd actually provide you (I'm sure you're not the only one) with more verbal cues ("leading with the right" or "step up with the left") and visual cues (raising the left hand and making a chopping action to indicate how it lands on the beat).
And seriously, no one really cares if you're leading with the wrong foot because everyone has been there, just keep watching and you'll pick it up.
I have mild dyslexia so the whole left and right is a mystery to me. It's not the easiest to give a verbal cue, for the class, which is the opposite to what my body is doing (I can't walk and chew gum at the same time but that's a whole other issue).
I struggle but I'm getting better at it and it's becoming easier so there is hope for you.