I'm a big believer in dedicated healers. They give you much more stability and longevity in fights. But you can't really expect to ding 80 and clear heroics. It's just not a reasonable expectation. 535 Defense and 20k HP is fine for regular runs, and starting out heroics. I think you may have been expecting a bit much too early.

Also, agility is not a primary stat for prot paladins. go for stam IMO. You have no real enchants except your boots. You can get 40 stam to gloves, stam to your sheild (iirc), Threat and parry rating to gloves, a kit with 30 stam on your pants. etc. If you're DPS is similar to your tank, it's defnetely a gear issue, spend time on getting gear and getting your DPS hitcapped, before banging your head against the wall of heroics.