It's always too early to quit.
The secret of getting ahead is getting started.
Be thine own palace, or the world's thy jail.
If you think you can do it, you can.
The dog that trots about finds a bone.
The most effective way to do it, is to do it.
Do not weep do not wax indignant. Understand.
I can, therefore I am.
Be gentle to all and stern with yourself.
Go for it now. The future is promised to no one.
I wrote somewhere during the Cold War that I sometimes wish the Iron Curtain were much taller than it is, so that you could see whether the development of science with no communication was parallel on the two sides. In this case it certainly wasn't.