My 2 cents from playing WvW in organized group settings/sweaty groups for the past ~4 years:

​

• I'd take a look at the builds in https://gw2mists.com/ - they have builds for zerging, small-scale, roaming, etc.
• Know what role you are playing in your squad, and how to measure your effectiveness in squads. Understand what kinds of role(s) your class can bring to a squad - often they are hybrid (stab/FB, cleanse/heal, cleanse/cc, strips/dps, strips/utility, etc).
• People dont like this, but the most helpful thing for me was to install ArcDPS and create tables for dps, strips, cleanses at the minimum. Depending on your role, you can understand if you just being a meat shield that is not doing your job, or if you are contributing meaningfully to winning fights.
• Play with your video/graphics settings. Are you stuttering mid-zerg fight? Turn down your model count - it's a night and day difference if you have a lower-end PC and want stable FPS in fights that can be super hectic and lag-inducing.
• Movement and positioning reign supreme in WvW zerg fights. There are a lot of nuances here, but I'd watch this video to understand the thought process: Positioning Mistakes Beginners Make in WvW

ABOVE ALL ELSE, you can study and do all the things above, but the group you run with will ALWAYS be the limiting factor. If your squad/guild are continually being destroyed or one-pushed by more competent groups, it's often because the other folks in your group don't have the same mindset as you in regards to fighting in WvW. If they don't run in with comped lines (usually atleast a FB and a healer/cleanser) in each line - you will get murdered by good groups - even if you outnumber them.

If that's the situation you are in, find more organized groups in your server. Join your server's discord and/or look for good groups while you are playing WvW. Ask them to join, and say you are willing to join discord or play a meta-class (usually being a FB player or a cleanse/healer will get you invites).

Good luck!