When you say you memorised it in a 5 hours what are we talking about? Are we talking in one sitting, or 30 minute sessions a day for 10 days? Or an hour a day for 5 days? And what does "memorising" mean. Does it mean you managed to play it once or twice without looking at the score at the end of a practice session, that you can go for a week without looking at it and still play it from memory, or that you can write the piece out from memory, dynamics, articulations, and phrasing included.
Also, "a week to play it in tempo without mistake and good technique"? How many hours a day do you practice and how many days a week?
And how regular has your practice been these past 18 years? All that is kind of relevant to the original question.
I would say that if you could write the piece out in full after a week of not looking at the score after a single 5 hour memorisation session, and could play it at tempo, without mistakes and with good technique after a week of one 15 minute session every other day, then you have made phenomenal progress!
If you memorised it after 15 20min sessions a day for 15 days, and play it at tempo after a week of 3 hour practices sessions twice a day, you've still made good progress… but after 18 years, things are going to move very slowly. At this point it might be worth not focusing so much on progress and just enjoy your playing for its own sake.