Don't be sad about hitting 80 o.o You can still have fun jobbing and doing stuff that lower levels can't do. Just try to buy some generic armor, alchemize it, and get in a party, I guess.
About the Warlock, that's how a lot of people feel during the boring 3x levels. Use Vampire Touch first to give Disease. Only after that, start casting the other gazillion skills because without Disease status, they will resist at least one or two debuffs all the time and that could kill you, especially if Impotent doesn't work. With Disease on, all of the debuffs will land on the first try 95% of the time. If they are a strong opponent, it would be best to use Stun first to interrupt them, and then Vampire Touch. So the skill sequence would go like:
1) Stun/Vampire Touch
2) Stun/Vampire Touch
3) Combat Raze
4) Medical Raze
5) Wheel Bind
6) Decayed (Bleed)
7) Blaze/Toxin (No dull yet)
7.5) Sleep is optional as an attack interrupter
7.9) Shadow Armor for overkill

Nuke and Vampire Touch over and over again
Takes a bit more finger work than other classes, huh.

But they are definitely not worthless.