Healers in this game are not impossible to play, instead they're vital for a party to survive.
Monks are generally believed to be the best healing class.
You're not mentioning mission so I can't comment on what exactly you may be doing wrong. I can only say that non-aggroed monsters shouldn't aggro for no reason. Either they get too close to you (patrols), you get too close to them, or it's because you attack them. Patrols sneaking up from behind may be confused by an auto-aggro, but it's because you're in their patrol path. Ensure you know where nearby patrols are walking so you don't attack a mob and then another one walks up to you, giving you a nasty surprise. In this case, you should of course kill the patrol first.
As for party composition for PvE, 7 non-monks + 1 monk is generally considered a bare minimum. In that case, you may do it, but it can get rough for the single primary monk to keep up in tough battles. In a 4 player mission, one monk should suffice though.
Your problem seem to be that your team took on too many monsters at a time. A monk is no divine being, it's only a profession, that just like any other has both abilities and limits. Your team's character levels doesn't matter if you take on too many monsters.
Next, some golden rules of GW gaming, not just Monk playing:
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You must always have someone call targets. A disorganized team will kill a Monk played by an elite gamer.
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You must always pull targets when the mobs are too large for your party to handle. Game experience will soon start telling you when a mob is getting too large for your party to handle immediately. Have a single ranged attacker run ahead of the party, fire a few shots to aggro them, run back to the rest of the party, and hope he got a more managable amount of monsters to handle for your party.
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Your party must always protect the Monk if s/he's in trouble. If your fighters are on some "front" a bit away from you and you're getting pummeled in another place, you'll be dead. They need to come back and help the monk first, so you can at least get time to heal yourself and the others. If you get some firepower on your attackers they might aggro those instead and suddenly the tide of the battle may turn as you'll get some time to heal yourself and them, and not just click like a madman on your healing skills to just keep yourself barely alive.
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You must be careful. 
Do you
know where that moving group of monsters are heading? Will they come back meanwhile you're fighting monsters? Is it better to attack a group of monsters from another direction instead of getting into a crossfire? Plan your moves. The monsters will stand like good puppies and wait while you decide what to do.

Run for your lives if you see a mob is way too much to handle -- run, run, and hope the monsters will let you go and move back to their positions and reconsider your tactics.
I have no idea how much experience you've got with this game, so I'm just giving you some very basic GW suggestions, as you must be doing something seriously wrong if you're under the impressions Monks are bad healers. The guilds dominating the ladder would disagree strongly.
