My discovery is the following in regards to these questions. Guilt is only useful for three things.
1. To learn a lesson.
2. To change a behavior.
3. To make an amends.
Guilt does serve to alert us that we have erred in some way which goes against our values. It is a signal flare from our conscious or unconscious mind that a behavior or thought is in violation of the standards which we hold true for ourselves. So, in this regard, guilt serves us because it is actively making us conscience. And since our mental and spiritual evolution is dependent on increasing our personal and relational awareness, guilt is an important vehicle for this growth.
However, the three things I listed above are all actions which should take place in due haste. They are typically not things we need to spend lots of time on or revisit often (if at all). Guilt should be a temporary state of mind, only used to affect the right shift in thought or behavior. Once we have discovered the error of our ways, made the healthy change in behavior, and apologized to the party we injured, guilt needs to go; immediately. Holding on to your guilt for one moment beyond acting on these three things is practicing in self-harm.
Let me say a bit more about this. Often due to a poor parental model, an overbearing or critical adult who was in our life (teacher, family member, clergy, etc.), or possibly from our religious background, we have gotten the false and destructive message that we need to carry our guilt. That, in some way we should pay a continual penance even after we have sincerely been contrite. I completely disagree with this. You are doing yourself and your relationships absolutely no good if you are living in guilt or shame. In fact, these feelings will surely create emotional, mental, and even physical sickness. They will also disconnect you from others and your healthiest self.
One of the odd things I notice is how generous my clients and peers are in their forgiveness of others, but how they often hold themselves to another standard. They can have empathy for another's wrongdoing, yet continue to punish themselves for a mistake or transgression that happened days, months, or even years prior. What is especially frustrating for me is when my clients, who express a Christian faith, miss (or misunderstand) the foundational teaching of forgiveness through Christ. In spiritual terms, they mistake the guilty voice in their minds as their own and not that of "the great accuser". This thinking goes against the very Spirit of the God they worship and plays right into the hands of what they would call satan.
Putting specific spiritualities aside though, and thinking just in terms of practicality, guilt is a useless and draining enemy of the mind. We all fail. We all have words and actions we wish we could take back. We all need forgiveness. That is our common humanity. It is inescapable. Forgiveness is the only way to restoration with yourself or others when a wrong has been committed. Without it, we are incomplete beings, forever longing for that graceful salve, and forever lost in our own private purgatory.
Last thing, you have heard people say, "I forgive but I don't forget." Then you don't really forgive. If true love keeps no record of wrongdoings, and I believe it does, than true forgiveness releases the person (in the case of guilt, that's you) from the destructive bondage of sorrow or penance. Again, once you have done the aforementioned three things, say goodbye to your guilt. You have taken care of it. You don't need it a minute longer. It is the enemy of your wellness.
