[2020] Runescape new combat, Why am I so weak now?

Your ability bar rotation can make a big difference to your DPS.

The optimal rotation will vary depending on what you're fighting and your equipment loadout, but if you're using Revolution++, this is a good place to start:

https://runescape.wiki/w/Revolution/Bars#Revolution++_bars

You can also right-click the cog on your action bar and choose "Setup action bar" to apply the default abilities for your current weapon to your current action bar (they're based on the optimal revo bars listed on this page).

You'll likely need to tune it a bit for specific targets, but I find it faster to start with that and tweak the defaults than drag every ability manually from the powers menus.

Another way of automatically setting your action bar is to Take from the melee, ranged or magic combat set dummies at the training camp north-east of Burthorpe lodestone. (IIRC their rotation's more suited to single targets.)

The Settings->Gameplay->Combat & Action Bar menus also have various toggles and settings that are worth a look to customise combat to suit your playstyle.

Binding weapon types to action bar presets to automatically switch ability bar when you wield a different weapon, for example, automatically triggering thresholds/ultimates with Revolution++, or automatically triggering the Regenerate ability to sink excess adrenaline into lifepoints when you're out of combat. You can set up combat to be very lean-forward or lean-back depending on your preferred approach.