i was slightly overleved from doing all the sidequests for about half of the game. later i left most of them for when i'd level a new character but i still didn't have any problems. are you doing dungeons from time to time? have you done the guildhests at least once? i don't know what to tell you otherwise. maybe you're rushing it too much. i assume you know that you're not supposed to get your first class up to 50 before reaching the end of story mode
Do every side quest you can find. Do every guildheist. Explore every region fully! Do the story and finish your hunting log. You'll have to fate grind a few time, especially from 47-49. The story picks up at 49 and you won't be far from 50 at that point.
Go to Costa de Sol and farm Fates till 35 then go to Coerthas till about 45 and then Northern Thanalan (Ceruleum Processing Plant) to 50. Takes a while but it's rather effecient.
I really wouldn't suggest Coerthas (for FATEs, at least) to a PS3 player. That place is waaaaay too crowded. I'd suggest just sticking to Costa del Sol until 38, then moving to Mor Dhona until the low-mid 40s.
For those players who don't speak Australian, we have provided an English translation of the previous scene. Do you want to replay the scene?