24 years on four before getting a fiver in Y2K.
I now play five string exclusively, and no longer own any fours. I do not mean to imply that you should do the same! Just saying that five is "just right" for me (I have no interest in 6 or more, either).
I know
exactly what you mean by muscle memory. What I would up doing was putting my fours aside to concentrate on five (I believe that's called immersion). After two months I was gigging on five, after another month I was actually comfortable gigging on five.

But times can vary: some players adapt much more quickly, others more slowly.
What also helped in the transition from 4 to 5: learning new songs and mapping them out from scratch on five string. That helped me get the hang of five string as a whole, and once I did it was easier to convert songs that I'd played for decades on four. Sometimes very little conversion is required, but usually the low B can provide greater convenience and creativity.
One final piece of advice: do NOT think of five string as "four string plus one". With a quality fiver the B string is playable well above the 5th fret, and of course 5th fret and above are in four-string land. It's not illegal to play fretted low E... to the contrary, use the B string whenever it's appropriate and convenient.