As others have mentioned, 95% of players have all classes at 80 and is only doing endgame content. I'd love to run through the levels again with a HoX or a Barbarian, but my slots are spent and I'm not buying more. Almost every quest is quite easy to do when it's marked green. Stay off the quests marked with red or group symbols unless you can get help with them.
As for playing the Barbarian in solo, key to success are keeping an eye out for which combos you are using, and having the highest base dps weapon you can at any time. Always use highest ranks of combos, and drop the ones you get at Tortage as soon as you can. Make keybinds for Clobber and Stunning Punch. As long as you are soloing, Blood Rage stance is the way forward. Make sure you feat Shock And Awe and Staggering Punch to improve your availability of CC's and also grant a slow to your Stunning Punch.
Use Staggering Blow to gain the +weapon damage, and use Butcher combos to nuke down the mobs.
Don't lose faith, it's a great game really but at seven years old, there is limited amount of players leveling, and given there is 4 big zones that offer quests at level 18-39 you'll be lucky to run into anyone doing the same quests as you at the same time you need to do it. And if you group with higher level players you get xp penalty for being carried. It's not easy to do quest grouping efficiently