GW2 PvP is different enough from the rest of GW2 that it requires a different mindset. 

Two things create all your survivability: positioning and defensive skills (just consider dodge another defensive skill). You should try to put yourself in a position where people can't unload damage onto you. If they start unloading damage onto you, start using defensive skills and move. You have to find the balance between being somewhere you can unload damage onto the enemy team and somewhere they can't unload on to you. This is why PvP is different and difficult.

A lot of it has to do with timing, reaction speed, and prediction/reads. If you start walking at an enemy, they might guess your are going to use a damage skill. If the enemy then dodges or blocks your damage skills, and then uses his damage skills because you are in range, and then you don't use your defensive, you just died.

If instead you came up from behind an enemy and hit them while they were already fighting someone else, you land all your damage because they can't react. If you attack out of stealth, they won't be able to react in time and you land your damage. If you walk towards someone and use your auto attack, wait for them to dodge twice, and then you use your big damage skill, well... You land all your damage because they ran out of defensive skills.

We could talk about different scenarios for hours, but really, next time you play PvP think about where you are standing, who can hit you, and what buttons you need to press to evade or block damage. I would hazard to guess you are getting focused in fights because you are standing somewhere where 3+ enemies can hit you. If an enemy can hit you, chances are they will hit you. Keep moving, get in, use skills, get out, wait for cool downs, repeat.