You must concentrate upon and consecrate yourself wholly to each day, as though a fire were raging in your hair. -
In concentrating on the day, you must leave behind the past. The past, for many holds regret, and should be remembered, not "lived in". The past can teach you what to avoid and let you know your strengths and weaknesses, but it cannot truly change the present. Damage from the past can affect you now, but in the now is where you learn to overcome your damage.
Guilt is a little harder. If you did something you feel guilty for, make your amends RIGHT NOW, and move on. If they are not accepted, it is OK. It is your responsibility to offer them up, not to ensure they are accepted. BUT, it is also your duty not to do that which will cause guilt in the future. Guilt is an excuse for bad behavior much of the time. Guilt can be overcome with attitude, this is one of the reasons I am posting this.
Fears, as damage from the past, can affect your present.
Fears are related to the future, born of the past. Fear can be both healthy and unhealthy. You must not fear that which is not related to everything. I am afraid of getting hurt in a relationship, is not related to all relationships. I am afraid of getting hurt from cancer, related to smoking. Smoking causes cancer, and has been linked and proven too many times to argue anymore. Do not forget that we ARE vulnerable, and that is the present. It is better said here.
So all in all, to live happily, do so in the present. This is the place that will create your trus destiny.