How long is "a long time"? Because over the year or so I gave sitename-passwords a try it popped up as an issue way more than 3 times, even after I started ignoring subdomains and instituted rules about always trying to use the main domain for big companies. There are tons of systems out there that use cross-domain (notably both of my banks and both of my schools), hidden-domain (i.e. log into an app or device where the parent domain isn't immediately obvious), or changing domains. Of course, these issues were nothing compared to trying to remember all of the variations I added to sate the enormous variety of conflicting password requirements (especially novelty requirements and passwords you don't get to choose).
I was in complete denial about how bad the situation was until I encountered some light teasing from acquaintances who didn't even know me that well -- just well enough to know I couldn't ever remember my passwords. I started keeping tick marks on my calendar and found a ~75% success rate, although the 75% was composed of the handful of logins I used every day and the 25% was "everything else" so the reality was that the password derivation system was failing for a strong majority of passwords.
I gave up and started using lastpass. No regrets, but many positive surprises: that time their network was breached but it didn't matter because they didn't use shit encryption, the automated password reset feature for big-profile leaks (they give you a "todo" list which is often as simple as clicking a single button next to each item), the ability to easily store serial numbers, "verification questions", and other nonsense, a general lack of guessing passwords several times before success, and an ability to dramatically up the cross-entropy of my passwords.
I might move from lastpass to a physical repository some time. But I am never, ever going back to a password derivation scheme.
I wish you luck. Or an eidetic memory. You'll need it.