I suffer from the same problem, I am what my trainer calls "weak on the left side" as well as my horse is.. he has trouble pacing with the trot on the left and keep straight, so for us both it's a work in progress.. On my right side I can post without even thinking about it, it just flows naturally for both me and my gelding because he can stay at a nice pace and even speeds up on the right whereas the left he slows constantly and struggles.
Practice, practice! all I can say.. The first time I tried doing it correctly, I couldn't even post on the leftside on the correct diagonal for more then 30 seconds.. Much less even be posting correctly.. Plus having to kick everytime my butt hit the saddle because he tried to stop or slow down.
But after much practice, I can pick it up correctly when sitting the trot first and without a foot in the stirrup... but it takes a lot of practice..