Get faster so you aren't the slowest in the medium lane!
Seriously though when I was between lanes I'd pick the quieter of the two, and if in te fast lane keep an eye out for people leaving the medium lane and move down there or if there was an increase in the fast lane and you're still the slowest move down.
The reality is that if you're still improving it will only be a phase and you won't have the problem for too long!
I had my first adult swim lesson a couple of weeks ago - was really useful to both be able to ask questions about my technique and have a few small pointers on things to improve has probably speeded me up by a couple of seconds per 100m
I think lessons are a better option than a clinic - people always seem to come away with far too much to think about to be able to make any meaningful change!