Correct, your refusal to do group content prevents you from completing the games main story. Thats no different than any other MMO that has epic quest lines or keying for raid content, you dont want to group up to get that accomplished then you dont get to do that content, its simple.
So yes you’ve chosen to make it a grind and have to level up by doing town boards and faction missions, YOU chose to have that be your source of xp.
As a side note, town boards give massive xp, I have not finished the main story line, nor a ton of quests in all the zones and im level 59, so clearly you’re missing something. I did tons of town board missions and farmed my pvp gear sets, minus the level 53 and 60 ones, but I’m level 59.
Refusing to do group content is your decision, so if the game is exponentially more difficult that is by your choice, its not “impossible”. There’s a guy leveling to 60 by only fishing, theres a guy leveling to 60 only through crafting, they arent on the forums complaining that its harder because that’s how they are choosing to play the game, you have options, you just refuse to use them.
I have no issue with people playing solo, in fact besides doing the expeditions, ive played solo the entire time because my group was on a different server than myself, I chose to solo. My issue is you complaining that its “impossible” when in fact it is not.
. So now I play as a healer in this game and yes i realise others look at me with the same scepticism 
