There is good and bad in every profession and I think you have just experienced the later. NOT TO WORRY. I would not EVER recommend using thinners for someone who styles their hair curly. This definitely reduces the curl result and definition. The stylist probably thinned the ends to remove the cutting lines that were visible after having your layered when dried straight. ...More >
I would recommend having a weave cut. By this I actually mean sections of your hair been taken as if to colour it when having weave highlights. Then each of the "weave" pieces in each section are cut at different lengths with scissors. This will thicken your thinned ends that have been put in previously.
It is really difficult and I can understand your frustration but when you have long curly hair the last think you want is normal layers introducing or your ends thinned.
The weave cut will help remedy some of your problem depending how far up your hair they went while thinning.