> My mentor was the second to object. He said I shouldn't quit without another job lined up because I'd miss out on leveraging my high salary when negotiating my pay at the next job.

I feel like this is objectively good advice though. At least keep an income stream.

...the highest I ever got paid was 15K euros, per annum.

When I read such numbers I go crazy, feeling completely worthless.

Imagine getting paid half of that amount he was getting paid! O.o