So, you got no power, magic, gold or height? Made a few mistakes? Don’t matter, bub, you gotta go do what you’re called to do anyway, even if you think you might not be able to it all:
The small can influence the world, so act even if you think it won’t matter.
I’d go with “Never meddle in the affairs of wizards, because once they’re aware of you, they’re going to involve you in their plans and gamble your life in risky plans with low success chances”. Look at what happened to Frodo! If it wasn’t for Gollum of all people, it would have all gone pear shaped!
Entropy. Everything always gets weaker, uglier, shorter lived, and so on. Tolkienverse history is a long slide of decay from superhuman wisdom and glory to mundane dullness and folly.
That no matter how bad things get, or how dark and hopeless they seem, giant sentient eagles will come to rescue you and your friends and make everything OK.