My wife and I are also trying to do this, and I suspect a big part of why we've been able to stick to it is the growing effective altruism community. We commit to donating 50%, post our income and donations publicly [1], and we know people would hold us accountable if we started backsliding.
Still, 20 years is much longer than I have experience for, so I definitely respect people like you who have been working at this much longer!
Yay! Thank you! As someone currently choosing to live very minimally and instead donate as much time as possible to the projects I believe in, I really am grateful to those similarly-minded people like yourselves working from the other end of the lifestyle spectrum :)
Still, 20 years is much longer than I have experience for, so I definitely respect people like you who have been working at this much longer!
[1] http://www.jefftk.com/donations