Guard my life, for I am faithful to you;
save your servant who trusts in you.
The interesting thing in this verse is that David knows that he's been faithful and he knows that he trusts in God. I think if I heard someone say this prayer out loud, I'd definitely think they were being a cocky Christian (CC) and I would therefore judge them haha. But David knows that he's been faithful...
Sometimes I feel like the non-spiritual events in my life dictate my relationship with God. For example, bad grades=depression=crappy relationship with God. And, by the transitive property, bad grades=crappy relationship with God. Or maybe something like school=studying=tiredness=no time for God=crappy relationship with God. And by the substitution property, we can replace those things with other crappy things in life (family troubles, girl problems, money problems, sin, etc) and it still works. But here's David, full of faithfulness and trust, even when the events in his life force him to beg for salvation.
And I love the word "save" because it means more than simply protection or providence. In order to be saved, you have to already be in some kind of danger. I can't be saved if I'm watching WWE on TV, but I can be saved if I'm in the ring and about to get Peoples Elbowed by the Rock.... I like how the verse doesn't say "keep your servant safe, for I trust in you", but it says "save your servant" ... David isn't asking for daisies, rainbows, and comfort. He knows that he's gonna have crap to deal with in his life. It's not protection he desires, but rescue.
Is your faith constant despite what's going on in your life? Are you asking for safety or for rescue?