Oh man, I wish getting back to riding was like bicycling! So not the story for me.
I haven't owned a horse for over 5 years now and have only ridden a couple of times during that period. I used to do free riding in the pasture both bareback and bridleless at the drop of a hat. Now? Not so much!
While bareback, I feel like I have no natural seat anymore and am overly focused on not losing my balance. That of course gets me nervous, which doesn't help my riding either.
So, it could be a whole slew of things thrown together for you - may or may not be one simple fix (although someone might pinpoint your issue(s) in pics/vids). I presume more riding time will help you out, build muscle memory, and of course, with more riding time, the more confidence you'll probably gain, but I totally understand that it is hard to get to that point!
Now, if the manpower was available to me, I'd like to have someone lunge my horse, that way I could focus solely on my seat and not have to worry about runaways, crazy dogs, w/e. I'm not sure how nervous you are right now, but perhaps someone leading you around could even build a nice base of confidence?