Philippians (4:8-9) (NASB):
Finally, brethren, whatever is true, whatever is honorable, whatever is right, whatever is pure, whatever is lovely, whatever is of good repute, if there is any excellence and if anything worthy of praise, dwell on these things. The things you have learned and received and heard and seen in me, practice these things, and the God of peace will be with you.
The purpose of a horse's bridle is to twofold: to hold the bit into place, and to transform the pulls of the rider into pressure on the soft part of the horse's mouth - and allow a few ounces of pressure to guide a massive animal.
But the bit and bridle rely on a fundamental principle: the horse moves in the direction its head points. Turn it to the left or right, and the whole massive animal turns. Keep it straight, and the beast runs true and straight.
For a horse, the bit & bridle are physical. Keep the bridle well-fitted to the horse and it won't dislodge... leaving the direction to the whims of the "dumb animal".
For a Believer, it is the Holy Spirit that guides us. But as a real horse can get his bit between his teeth and ignore the guidance given to it... a Believer can ignore the Holy Spirit. Keeping in prayer and humility and we won't ignore the Spirit, leaving our direction to the whims of the "dumb sheep astray from their Shepherd".
The Lord has given us many, many ways to run true and straight. I am partial to Philippians 4:8-9, where we are given the guidance of dwelling on those things that are true & honorable & right & pure & lovely & of good repute ... that are excellent and worthy of praise. Christ is the very embodiment of all of those things.
When I let my thoughts sit steadfastly on the Good attributes of a person, or on those perfect qualities of Christ, then my life is aimed True and well. When I follow either the world, or my own sinful nature, and dwell on the negative... then I get mean-tempered, petty and selfish - and then I fail to represent Christ as He deserves.
And then I must repent and begin again to dwell on These things...