I think that lone verse in Proverbs about guarding one's heart is far over-used, in the too-narrow application of romantic love.
NIV: Above all else, guard your heart,
for it is the wellspring of life.
ESV: Keep your heart with all vigilance,
for from it flow the springs of life.
KJV: Keep thy heart with all diligence;
for out of it are the issues of life.
Plenty of chapters in Proverbs provide no context, and thus might be harder to interpret in specific application, as they are simply a collection of wise sayings. This one has a little, though, being framed as a father speaking wise counsel to his son. Yet it's funny how this verse gets quoted in teaching to girls more than any other it seems. Not that it has no application to young women, I'm just sayin...
The key though, here, is defining 'heart'. Looking at other uses of this same word, it's not simply the seat of feelings, where you get the warmies for somebody romantically. And in this particular instance it's defined as the source of the wellspring of your life. That's a LOT more than just romance, and I wonder how little it actually has to do with romance.
I think a more accurate teaching would stress not the avoidance of pain or keeping others from hurting you or affecting you emotionally, but rather maintaining your heart - your innermost being - in the way of the Lord. Because if you let the world gain control of your heart, what springs from you will not be good.
Matthew 15:18 NIV
But the things that come out of the mouth come from the heart, and these make a man 'unclean.'
And while you're busy doing your part, you can rest assured God is doing His:
Philippians 4:7 NIV
And the peace of God, which transcends all understanding, will guard your hearts and your minds in Christ Jesus.