What am I doing wrong?

Just to echo schnorki, runewords like Oath used to be ladder only, but not in D2R. Since you are playing online, a

Mal rune and an

Eth base should be quick and easy to trade for. In your position, I would prioritize that and a source of

Decrepify on your merc above all else

Edit: or since you are talking about trading for a

Lo to make Fortitude--definitely make Grief first then. Again,

Zeal is all about your weapon. Making a Fortitude to amplify your damage won't do much if there isn't any damage to amplify

Hit me up on bnet, Mohammad. I could share some gear!

It really came down to itemization. Had a kind soul give me a Grief among other items and I am pushing well into the third act.