I have to agree with luxlunae that it'd be much more forgiving with an actual tank/dps/healer setup to start off with. AoE healing can only heal so much so fast (especially when you're undergeared), and if everyone in your party is taking damage then that's probably no good until you get the gear to soak up that damage (or heal through it).

It also helps to have a flexible setup in your software as well. If you're trying to jam everything onto one single mindless macro rotation (healing included), well there's not much flexibility to be had and I'm going to guess that there are CDs and other abilities that could be used at a much better time than just hoping they fire off in the rotation at the right time or ignoring them altogether because they're "too hard" to micromanage.

A lot of people get very comfortable (or lazy) with the ease of dungeon leveling and once they hit the top level where stats are normalized, it becomes a rude awakening and they're, more often than not, unprepared for it.

You also said your iLVL is ~435. Crafted PvE (iLVL 450) pieces on the AH should be dirt cheap at this point in the expansion unless your realm is a ghost town.