Why cant i get past lvl 55?

Leveling from 50-58 in my opinion is one of the toughest periods you will experience. Starting at level 50 I like to use this route

Felwood>Western Plaguelands>Winterspring>Eastern Plaguelands

Since the AQ patch I hear at higher level 50's (57-58) Silithus is a decent place to quest as well, although I havent personally tried it.

Just stick it out and strive for 58. Once you hit that go to outlands where its 150% easier to level.

Good Luck

Just keep grinding i suggest doing quest from eastern plague lands and also western plague lands. u will get a whole lot of killing and collection quests. thats all i did and it took me about a week of playing to got from 49-58.
(note that this is just an estimate of actual played time).
also leave your toon at an in when u are not playing just to get the double experience bar it helps alot when doing the quest in eastern and western plagued lands.

The way I always level 45-58 is like this.

45-47 Tanaris
47-49 Searing gorge/Hinterlands
49 - 52 Ungoro
52-53 Burning Steppes
53-55 WPL
55-58 EPL/Sillithus/random dungeon run

If you arent afraid to travel, its pretty quick.
Nowdays I ignore almost all of Northern Kalimdor, just seems to far away to be worth going to it. I normally hit up Ashzara ONCE for a toon's level 50 class quest chain that leads to ST, but then just ignore the whole northern hemisphere of the continent.

If you are level 55 now, I suggest running to Eastern Plagelands, running to the far western part of the map, near the river that splits EPL/WPL and getting Fordring's quests.

It is 3 "grind" quests. You need to kill a total of about 150 monsters to finish those 3. They can be found all over the zone and done while you are doing the other quests in the area. They offer great quest exp and chain into more quests.

If all else fails, Sillithus is PACKED with quests that you wont be interupted while doing. It's a dead zone. Just like before AQ was implemented... there is hardly anyone there.. ever. Unless its people leveling mining.

Also, to make it easier for you. Dont forget at level 57 you can buy outland greens off the AH. Makes for much faster leveling with just a few peices of it.

Grinding is rarely better than questing. Unless you are 1-2 bubbles from level and have to fly 30 minutes away to continue questing.

It's best when you can run in some grind quests and benifit from both methods.

Also demonology is def not your best bet for leveling.

Affliction is widley known as easy mode. With the VW buffs latley you should be able to dot up 3-4 mobs while he tanks them and you look for more to pull to him.

Dont get me wrong, Im a fan of demonology, its just not an optimal leveling build.

Sorry for the double post, wish we could edit >_<