| Quote #1
I know it for truth: it is in a warrior
With "I know it for truth," the speaker signals that he is about to give what's called a "gnomic" statement, a popularly-held opinion about how a person ought to behave. These statements are also sometimes called proverbs or aphorisms. They're expressed as being absolute truth (like the one here), although they're usually just the cultural norm of one particular group. Anglo-Saxon culture had a long tradition of gnomic sayings collected in catalogue (or list) form, many of which expressed a similar sentiment to this one: that it's best for a person to contain his emotions within himself.
| Quote #2
Weary mind never withstands fate,
With another gnomic, or proverbial, statement, the speaker gives a possible reason why it's better for a warrior to lock his emotions inside of him: because such emotions do no one any good. This bit of wisdom implies that communication or speech ought to serve a purpose: what you say or express should be useful, or you shouldn't express it at all.
| Quote #3
So he knows, who must of his lord-friend,
In addition to regretting the loss of the wealth his lord provides, the speaker also misses his "lore-sayings," or teachings. We don't necessarily think of war-lords as founts of wisdom, but an ideal Anglo-Saxon lord needed to be as wise as he was brave. In fact, one particularly hated Anglo-Saxon king was called Aethelred the Unraed, or wisdom-less.