Will He guard me?

Have you ever had one of those days when you feel like screaming, complaining, or crying for no apparent reason? I know that at least all females will answer yes to that, and maybe some men as well. It really is a scary feeling, and rather a nuisance. One little thing triggers one little thought that in turn ruins my attitude for the whole day. This is not how I want to live.
The Bible tells me to guard my heart and tame my tongue. I can tame my tongue with too much trouble - I shut my mouth and keep completely quiet (sometimes to the extent where I build up pride for my "matureness" in handling myself well). But guarding my heart is something which I find very difficult. Especially being in the city, there are many things that come up which threaten the purity of my thoughts, and in turn, my heart. How do I guard my heart from shame? Jealousy? Apathy? Bitterness? Lust?
God promises to keep us safe and to bring us victory. This morning, on my way to church, I read Psalm 91. Read the whole chapter and you will see the promises. Here are a few which I find especially encouraging.

Ps 91:3"Surely He shall deviler you from the snare of the fowler and from the perilous pestilence. He shall cover you with His feathers, and under his wings you shall take refuge; His truth shall be your shield and buckler."
 91:13-15 "You shall tread upon the lion and the cobra, the young lion and the serpent you shall trample underfoot. Because he has set his love upon Me, therefore I will deliver him; I will set him on high, because he has known My name. He shall call upon Me, and I will answer him; I will be with him in trouble; I will deliver him and honour him."

Ps 93:4 "The Lord on high is mightier than the noise of many waters, than the mighty waves of the sea."