So may times I have seen both youngsters and middle aged people book one lesson a month and then been surprised that they aren't achieving.
That's essentially what I do, so I implore you to define "achieving." Surely steady progression in the course, passing exams and gaining more aviation knowledge than last time is an achievement?
I'm happy with my rate of progression and "achievements" so far, and if it weren't for legal issues, I WOULD commit more time to the course, but that's not possible, so I make do with what I have.
I say that after getting 92.5% in the Aviation Law exam today and going solo.
To be fair, I did consider the pace to be an issue, in forgetting procedures and checks that I only use once a month, but after REALISING that, I simply cemented them even further, and I have no problems with the frequency of lessons at the moment. The key is to keep on top of everything you have done to date so you don't forget for next time.