What contraction "I'd" would be if it's not contracted?
I'd not work harder, but I've learned a lot since then.
I'd normally means either I would, or I had. However, I can't make either of those work in this sentence as I can't work out what the sentence is trying to say.
Is the writer trying to say that he used not work hard, but he's learned a lot since then (whenever then was) so now he works harder?
The only way I can make "I'd not work harder" work would be in something like this:
Q - What would you do if we paid you more?
A - I'd not work harder (I wouldn't work harder) but I'd go on a lot of holidays.
The second half of the sentence is what makes it very difficult to understand. Can you post the whole section that your quote came from so that we have some context?