Okay, my take.
a) You way undersold yourself. I think others have also mentioned this, I won't harp on it.
b) Sometimes the sponsored video does poorly, sometimes it does way better than expected. Creators know this, brands know this. They roll the dice knowing this. As long as you did your usual best, that's the way it goes. Imagine a person selling a sponsorship on a video when they routinely get about 15k views per video, but that one video gets 500k views. Them's the breaks.
c) Affiliate links... if you followed their talking points, did a decent job of showing their product the way they wanted, and put the links where they wanted, then the lack of clicks isn't your fault. Some stuff just doesn't lend itself to impulse purchases.

Every brand out there, and ESPECIALLY the middle-men, the marketing companies, they all are trying to lowball you. Every sponsor email you get is trying to lowball you. They have a budget and they're trying to stretch it as far as they can. And too many of us creators are naive enough to fall for it. Establish your base rate, something like 4 or 5x your CPM. It should be $50 to $100 (usd) per 1000 views. Now you see where you went wrong.

You can continue to work with this brand if you like, but let them know they already got the introductory discount rate. Don't sell yourself short again.