When I people on this board talk about guilt, I notice they are often talking about completely different things. So I looked around the web and found an article called "The Definitive Guide to Guilt" posted on Psychology Today website on August 11, 2012. Here is a summary:

Cause #1: Guilt for something you did. You actually did something wrong.
– Feeling guilty for an action deserving of remorse is normal; to not feel guilty may be a sign of psychopathy. The problems occur when you ruminate over this guilt. An action in the past cannot be changed, no matter how much you wish it would. Accept the fact that this happened, apologize to the person or persons you harmed, and then figure out how to avoid committing the same act in the future.

Cause #2: Guilt for something you didn’t do, but want to. You’re thinking about doing something that deviates from your own moral code or that is dishonest, unfaithful, or illegal. Like Jimmy Carter, you may have mentally lusted after someone other than your spouse or long-term partner.
– To cope with this type of guilt, you can recognize that you have these illicit thoughts, accept them as part of who you are right now, and then, commit yourself to changing your behavior so that you don’t follow through on them.

Cause #3: Guilt for something you think you did. If you think you did something wrong, you can experience almost as much guilt as if you actually committed the act. One typical cognitive source of guilt is the magical belief that you can jinx people by thinking about them in a negative or hurtful way.
– Before you start accusing yourself of wrongdoing, make sure that the wrongdoing actually took place. If you’re distorting your recollection of events to make you seem more at fault than you are, it’s time for a hearty dose of reality testing.

Cause #4: Guilt that you didn’t do enough to help someone. Perhaps you feel you should have done more to help a friend who is ill or who is caring for an ill relative. You may have given hours of your free time to help that person, but you find you can’t continue to do so.
– It’s important to separate your desire to help from the guilt you fear will overwhelm you if you don’t. Acting out of guilt can only drain you further and ultimately make you a less effective helper.

Cause #5: Guilt that you’re doing better than someone else. The experience of survivor guilt is experienced by combat veterans who outlive their fellow troops, people who lose families, friends, or neighbors in disasters, and those who make a better life for themselves than do their family or friends.
– The only way to cure yourself of survivor guilt is to remind yourself of how proud, glad, and invested those who love and care for you. Remind yourself, as hard as it might be, that your own failure will not help bring someone back to life, nor will it make others who love you feel better about themselves.

For Cause #2, another possibility is your moral code adjusts so that the action you're thinking about doesn't seem wrong any more. On the face of it this seems a risky and wrong move, but on the other hand it's something that happens to almost everyone, gradually, as they grow up.

For example, some religions forbid members of the opposite sex from dancing with each other; few others would blame someone who decided it was not an offense against God and did it anyway. Disobeying one's parents? Sometimes there's no other way. My wife used to feel a little bit guilty about having smoked pot in college, but then she discovered that it helped with the pain while she was recovering from surgery, and it was a victimless crime in any case.