On the other side of the worry, I'd be wary of just using a Password Manager to handle ultra-strong (i.e. as random and complicated as you can get) logins to other sites. You lose access to the PM, you've lost access to everything it's remembering for you... (It's bad enough with losing single logins, as above...). Even if you're not worried about that handy store being half-hinched (maybe your PM master-password gets discovered, even if the PM is utterly legit).
And if you have a plaintext file with obvious passwords in it then you have the same issues (either losing it all, or it spilling).
I won't advise anyone to a specific approach, but perhaps combine security (safely away from others) and security (safe from loss by yourself) by using a couple of different approaches, other than pure memory, and anything 'plaintext' (.txt file, post-it note left deep in the back of a drawer, whatever) is further obfuscated (like reversed order of characters BUT NOW I'VE SAID THAT,VDO SOMETHING ELSE..!). And never spell out what each is for ("Yahoo mail: <1revethaW>; MyBank PLC: <2revetahW>; ...") but obfuscate that (e.g., but definitely think about doing it better for you, the first thing you actually bought online was cheese, so "MyBank PLC" is "Cheese"; You once had a long correspondance about rabbits, so Yahoo is "Bunnies"), assuming you can't think of better ways that work best with you.
Ditto think about how you choose your passwords. If you're a "pa$$worD1" type person, barely more than the minimum (say) eight characters, both cases, digits and 'special' characters, then "1Drow$$ap" might be a particularly bad mnemonic (from the POV of being not being detected). Maybe you could get away with the other way round (written and used switched), to thwart casual colleague/family-member intrigue, but you need to consider all the possibilities applicable to you (and what/who you're worried about). But if you faithfully wrote down your PM's 'suggested' 30-character Extended-ASCII mix of random characters, that it autogenerated, then you can probably set it down in almost any (non-vanilla) layout and you won't be giving much of a clue to anyone who doesn't know your method...
Remember to update things when you change passwords, and add new ones before you forget. And if you're inclined to want others to ever have emergency access (next of kin?) then perhaps arrange a method of conveying only those passwords you want them to have that you can convey the details of the finding of from a hospital bed or (in extremis) as a "dead man's handle" delivery or something where things are worse than that (there are numerous ways to accomplish this).
Really, think about what works for you. I'll tell you now that I'm not actually as thorough as all that myself. But I've thought a lot about it anyway, from a professional (as well as philosophical) perspective. I'm probably more lax about some things than I ought to be, with potential for unintended third-parties, and in others I'm far more 'one physical failure from oblivion' than I should be - insofar as various online presences are concerned.