A troubled heart is something you "let" happen. This means that neither circumstances nor other people may be blamed for this condition. If we find ourselves with a troubled heart we may be certain it is something we permitted to happen.
What then is the cause of a troubled heart? The verse is very clear - failure to believe God. Therefore, the next time we find ourselves with a troubled heart we should accept full responsibility and move our faith back where it belongs.
HT: Mason Vann.