To be fair, Yasuo is one of nasus's favorite matchups in top lane.
The problem is that outside of your shield, you're one squishy boi, so Nasus isn't gonna have any trouble dealing damage to you while it's down. (The general guideline to that matchup is to just all in everytime yasuo gets near nasus - he wins at basically every stage if you aren't massively ahead since he can just ult and go for q's every 2 seconds) .
General counterplay to yasuo is w -> e -> q's- you're heavily slowed, you lose your shield, and you lose quite a chunk of armor while you're still inside spirit fire. Doubly so if you don't have any minions to dash through. If you don't have mobility you're actually quite an easy target for Nasus to all-in, especially if you don't have R or summoners ready.
I have no idea how much gold nasus had, since oftentimes you tend to measure it more from CS or tower plating than actual kills, but assuming he has triforce done he is certainly a force to reckon with if you aren't building tanky.
Best general advice I can give is to NOT stand and fight. That's exactly what he wants you to do so that he can wither you and then q you to death. His R gives him a massive amount of stats, so don't underestimate him unless he's significantly underfed. Kite him out or use minions to take short trades and poke him down.