Mortal Sin

This past week I've been asked several times what constitutes a mortal sin. I know that a mortal sin must involve grevious matter, sufficient reflection, and full consent of the will. Is this really clear though? Does this really provide us with an answer? Is there a clear cut definition of what is grevious matter? What constitutes sufficient reflection?

Don't get me wrong. I definitely agree that there are mortal sins which break our relationship with God. I'm in full agreement with the Church's teachings on mortal sins, I just think that sometimes it can be difficult to discern between a mortal and venial sin. I mean, there are some sins that are obviously mortal such as apostasy, murder, and abortion. But then there are others that aren't always so clear...

I suppose that if you're going to confession on a regular basis and confessing all your sins that you can remember, both mortal and venial, it shouldn't really matter. But then the problem is in the reception of the Eucharist. If you've committed a sin but your not sure if it's mortal or not, should you refrain from reception of the Eucharist? On one hand, one doesn't want to further offend God, yet on the other hand, being overly cautious could lead to scrupulosity.

According to the EWTN's article on mortal versus venial sin:

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A serious, grave or mortal sin is the knowing and willful violation of God's law in a serious matter, for example, idolatry, adultery, murder, slander. These are all things gravely contrary to the love we owe God and, because of Him, our neighbor. As Jesus taught, when condemning even looking at a woman lustfully, sin can be both interior (choices of the will alone) or exterior (choices of the will carried into action). A man who willfully desires to fornicate, steal, murder or some other grave sin, has already seriously offended God by choosing interiorly what God has prohibited.

Mortal sin is called mortal because it is the "spiritual" death of the soul (separation from God). If we are in the state of grace it loses this supernatural life for us. If we die without repenting we will lose Him for eternity. However, by turning our hearts back to Him and receiving the Sacrament of Penance we are restored to His friendship. Catholics are not allowed to receive Communion if they have unconfessed mortal sins.

This doesn't tell me anything that I didn't already know though. My question is on the gray areas. How do we know?

For instance, I often wondered if pride was a mortal sin, and then I came across St. Thomas Aquinas' discussion of this is in the Summa Theologica and found that only perfect pride was a mortal sin. It's a very good article to read on pride by the way.

Anyways, I'm simply musing about the gray blurry area. Perhaps I'm being scrupulous. Perhaps I'm being arrogant. I don't know.