People who play pvp regularly or exclusively will min-max their builds, so even if you're the same real level as them unless you're l337 you'll likely die a lot. Combat strategy is different in pvp than in pve and for levelling, and people will have different builds often times.
For the most part, gear set bonuses don't count in Cyrodiil (I think there are a couple of exceptions). Vampires do really well, werewolves not so much. Nightblades and Sorcs seem to be where it's at, and specific builds at that.
Magelight is very useful to have in Cyrodiil. When morphed to Radiant Magelight it shows people in stealth, and reduces the damage you and allies take from stealth attacks.
Honestly though, you will die. That's what pvp is. Even the best will die a lot, because there are a lot of the best and they can't all win against each other all the time; someone has to lose. But the more you play, the more you'll learn a few strategies, the best places to stand in big battles, how to decide whether it's worth attacking someone or stealthing and trying to just sneak by, and so on. And yes, of course, the more morphs and passives you unlock the more impact you'll have, but it's not necessary to be lvl 48 in order to do well in there.
Spend some time exploring your own faction region (especially if you're currently in control of it). There are loads of dungeons and little towns, loads of hostile npcs to kill, shards to collect, lore books, etc. Be careful going into dungeons in enemy faction territory, because you're more likely to encounter enemy players there, and there are few places to hide.
I hear they're talking about bringing back forward camps (or have they already done it?). They were a thing you purchased from a vendor in your faction area, and you could set it up out in the field allowing anyone from your faction to travel to there (and I think rez there too, but not sure). Enemy players could destroy your forward camps. Not sure why they got rid of them, presumably a balance issue.