You're thinking about CoF Storymode, if all your experience comes from SM dungeons you'd better hold your horses a bit.
(granted CoF explo isn't very hard either but ok)
The thing most people seem to forget about this game is that it's built around your team and team effort so ofcourse you on your own will have a hard time to keep everyone alive and what not. Especially if you take into account that most lethal dmg in gw2 comes from obvious things which should/can be avoided by dodging or simply strafing.
Another issue is that tanking=/= healing. In a trinity based mmo you have 3 roles, dps, tank and healer. If you're trying to "tank" by healing your team you're doing something wrong.
A better way to look at "tanking" is simply doing massive dmg in melee whilst knowing how to dodge so your team doesn't have to or using cripple/immobilise/daze and kite the boss (this only works if everyone is ranged since the agro system takes proximity into account)
All issues you've described so far come from lack of knowledge, be it fight mechanics, class mechanics or how to compose a team.
Being able to do a dungeon with 5 of the same class doesn't mean all of those same class were 30/30/10/0/0 specced using a Staff and the same utility skills.
If you see an ele who's built for condi dmg and uses fire combofields to help keep burning up whilst you're a guardian who also grabs the fire combo field to help with dps, you'd be better off with dropping that skill for something else, sadly not many players are open to sugestions or skill advice.
Lastly I'm going to make a point with which many people seem to disagree: toughness/vit/healing are overrated and very useless. I can expand on this but I think already have somewhere on the forums.
None of the dungeons are very hard and that's the major issue I personally have with GW2.