Your post doesn't really let us know what happened and why you are so mad. If it is indeed that you joined some zergers and got swamped by their leftovers there are 2 options that immediately spring to mind.... firstly don't group with them again, and when you do join a group ask if they intend to zerg leaving a trail or annoyed very much alive mobs in their wake. If the answer is yes join someone else, or you can do what i did and pretend the game is a one player game for the most part. I've played as a casual player from the launch, been in the same guild since just after that (there are now only 3 of us left) and I solo all the time. I'm at the point where I can keep up and contribute healthily on my main now, except on the higher reaper levels. But I still solo because I because I can dictate the way I want to play in that particular moment. Sometimes I'll flower sniff (especially content I'm not that familiar with) and sometimes I'll race to the end like the hounds of hell are hot on my heels. And then again sometimes the hounds of hell are hot on my heels and I see a nice flower....
If it's a difficulty level thing, then just drop a level or two, no shame in that. I can solo most content on R4 or R5 fairly comfortably but I quite often do quest on Elite instead, if I'm pushed for time or just "not feeling it" today. Sometimes I'll try R6 or R7 and regret it half way through, recall and start again on a lower setting. Other times I'll get all stubborn and blow a load of cakes and spell point pots and other resources finally finish and weep over the wasted supplies. Even though I've had hundreds of cakes and spell point pots for years and only ever use any on my "pig headed" days.
If you are already playing solo, or on the lowest difficulty, and some changes have made your previous strategies ineffective.... slow down a bit. Take a little more time and work out new strategies that work for you. One of the beauties of the game is the way different strategies and builds and classes work well for different people. Most of the "elite" players of the game list (or have listed until some rebalance or another came along) Sorc, Paladin, Barbarian and Druid as the most powerful classes in the game... I can't play any of them. I'm hopeless at them all.... well if we are being strictly honest I may have made a rescent breakthrough on the Paladin front, which only goes to show that there isn't a problem with any of those classes it's me and the way I'm trying to play them.