Two things to try. My OTTB had a VERY thick mane..unusual in and of itself. When it got long he had equal amounts on both sides of his neck (it would part down the middle) and he had enough mane literally for two horses. What I would do is pull form the right side, not shortening overmuch, then, comb over the other half and thin some more.
If it got too long, rather than pull I would actually shorten it SLIGHTLY with clippers and then pull from underneath to the desired length.
The idea is to thin by pulling from underneath or the middle rather than overly shorten. A short, thick mane is going to stick up like a Mowhawk :) I found this out the hard way on my very first horse who was a Morgan/Welsh cross with the thick mane from Hades...in any type of wind it would stick straight up from the crest.