In order to forgive yourself, I believe it is a process. First of all, what helps in letting go of guilt, is to understand that all of us are a changing personality. Seeing the changing nature of ourselves may help in understanding that from time to time we will get lost in the hindrances, if we are not fully enlightened. And from time to time, we will make mistakes in our actions, speech and thoughts. Compassion for ourselves is to look at who we were when we made the mistake. If it was not intended, then that will help a great deal in letting go of the guilt, because we see that Dukkha for others wasn't something that we wanted. So, we look at that person who we were when we made that mistake, we try to see what was affecting the mind of that person at that time, what hindrance were we lost in? Was it greed, was it aversion, was it restlessness and worry, was it sloth that went into aversion, sloth that went into worry and fear, or whatever it was, was it doubt? And we try to remember how these hindrances affect us when we are observing them in our meditation, how they bring suffering to ourselves, so we see that the person who we were who caused suffering for others was suffering from wrong views and from difficulties. Then a more Compassionate understanding of that person may arise.
But it is not enough just to stop there. We also have to make a resolution at that time to try to increase the powers of awareness, increase the powers of Compassion in ourselves, so that we do not repeat these things over and over all of the time. If we have actually seen a change in ourselves, then that can help, and then we can try to forgive ourselves, and ask forgiveness of others to whom we have caused the pain. Sometimes people find it hard to ask forgiveness of others, but it is very freeing for the mind if you are just truthful. Truthfulness, understanding our own imperfections and our ability to make mistakes, and learning how to forgive who we were.
Also at that time it may be helpful while we are contemplating this, to universalize it by bringing to mind other people who also get lost in these types of difficulties in the world, so we are not just focused on "me, me, me, I made this mistake, I always make this mistake, I wish I was better." Compassion for the person we caused Dukkha to, going out to others in the world who make similar mistakes, and trying to see how those mistakes are universal. Then trying to forgive the person we were, and making a strong resolution to avoid making these mistakes in the future. Unless we make a resolution to try to avoid them, then we may think "Oh, well, I can just do it again, and go through the process of Compassionate forgiveness." But this is a very painful process, unless we make a resolution to try harder in the future to try to watch ourselves in these situations and bring more control to them. If we didn't understand the situation, if it was something that we did without the understanding that the results may arise in this way, then we try to learn from this situation and try to figure out how to avoid similar situations in the future, and be willing to accept our Kamma.
It is important to understand that there is a difference between moral shame and guilt. Moral shame is necessary for us to try to see what actions in the past brought suffering to ourselves and others. And if it was through a lack of understanding and virtue, then we try to see that the action we did was wrong, but the person who was doing it may have been ignorant at that time. That is where we can prevent guilt from arising. Being more interested in the present and the future, rather than agonizing over the past and what has already happened. Because we cannot change the past, we can only learn from it, and try to increase our understanding and Compassion from it.