As far as endgame goes, I’ll first repeat what Arena Net was always saying to hype up the game. “The whole game is endgame.” What this actually means is that there is no sudden shift in how the game plays or what you do once you hit max level. There is no point where you suddenly step into a new game-mode, or get some new goal or mechanic that changes how you play the game. For example in most MMOs you level up doing quests and story content, then you hit “endgame” and are just expected to repeatedly grind the same dungeons or raids for better gear that will soon be outdated when the new patch brings a new raid and even better gear.
That doesn’t happen in GW2. Instead the things you’ll be doing at max level are the same things you’ve been doing the whole time; dungeons, story instances (shifting form personal story to living world), exploring the world, crafting new items (you’ll go from crafting stat-gear to collectable cosmetic items like backpacks and hard to acquire weapon skins), doing PvP if you’re into that, and fighting big open world “raid bosses.” The only real difference is that once you get to higher levels you can access higher level areas wherein the dungeons are tougher, the open world bosses are bigger and require more players, the crafting materials are harder to get, and the jumping puzzles are more challenging. Occasionally Arena Net will add new high level stuff to those zones as well, such as the recent “Great Jungle Worm” boss or the not as new Southsun Shores area.
As such “end-game” isn’t the same here as in other MMOs. In most MMOs endgame is really nothing more than repeatable content for progressively higher statistic gear. While in GW2 end-game is simply continuing the world’s story and (in some cases) dealing with progressively more complex or difficult content without a statistical reward or buff to eventually trivialize it. I guess in the end that is what upsets so many people about GW2 end-game; they are used to systems wherein their characters are always gaining more power, but in GW2 your characters stay more or less the same (with only minor changes through new traits or skills at rare instances) while YOU are expected to become more skilled and the community more organized to compensate for the increasing difficulty.
Oh, one more thing. Buying the past Living World story missions. You missed season one, and it can't be replayed. A lot of people complained about that, so Arena Net has given the ability to replay all of the chapters from season two. All you have to do is log in during the two-week span when the chapter goes live and "activate" it free of charge, and you can play it whenever you want from now on. However if you missed a chapter you will be expected to buy the previous ones with gems. You don't need the last chapters to get the new ones (I think), but if you want the whole story that helps. And yes you can trade in-game gold for the gems you need to buy those chapters, and you can do so at any time. I suggest just activating the living world chapters as they come and focusing on personal story/dungeon story right now, because the Living World takes place after it. Spoilers and all that. As to what happened in season one...well I'm sorry to say the wiki will be your best bet for that one.