When you hate someone for a prolonged period of time, it turns into resentment. Old resentments colors how you view the world and turns you into a pessimist who always finds fault with everyone and everything. It stops you from living the life that you are supposed to, and makes you cynical and sullen. You tend to alienate people and become unsociable and distant. So the question you need to ask is "Should what has happened in the past be allowed to taint your future?" If the answer is no, then you need to know how you can let go of old resentments.
Acknowledge your Pain
The first step in healing yourself is to admit that you are hurt. Trying to be brave, isolating yourself and withdrawing into a shell is not going to help. You need to openly acknowledge your pain. Yes, life is unfair and whatever has happened is not your fault. You need to talk about your feelings to a friend who is non-judgmental and whom you can trust. Airing out your feelings will help you in ultimately letting go of your resentment and pave the way to forgive the person responsible.
Sometimes it becomes difficult to open up about our feelings to even our best friends. But burying our hurt and pain only breeds resentment and hatred which makes us bitter and angry. One of the best ways to air your feelings is to write it down on a piece of paper. Think of it as word vomit that you expel from your system to feel better. Writing down how you feel is deeply cathartic and will eventually help you to get rid of your negative feelings. If there is a certain someone whose words and action have tormented you, then why don't you write a letter to that person stating how their behavior has hurt you. You don't need to send them the letter. Just putting your thoughts into words will help you heal.
Stop Holding on to the Past
Sometimes our anger and our hatred is so deep-seated that it consumes us and becomes a large part of who we are. We feed off from our resentment and know of no way of letting it go and think of nothing else. It becomes so much a part of our mental makeup and personality, that we fear that if we let it go, we would have nothing tangible to hold on to. We feel better about having someone to blame for our current situations and problems. We become detached and unsocial, so as not to get hurt and disappointed again. But we need to make a conscious effort to let go of our resentment and anger, so that we are ready to experience the rest of our life.
Find your Passion
One of the best way to get rid of past bitterness is to get yourself involved in new things. Try to identify activities that will invigorate you and make you happy. Maybe a kickboxing class or a dessert making class is just what you need. Maybe you have the potential to become a painter or a dancer. Uncover your hidden talents and passion and nurture them. Immersing yourself in new activities will get your mind off the past and you will have a lot of fun too.
A lot of times we like to crib and complain about our hard life and blame it on karma and luck. But what has happened has happened, and you cannot go back into the past and change it. Your best bet is to live in the present and look forward to the future. Instead of complaining about how hard and unfair life has been to you, think about the things in your life that you are grateful about. Maybe you have a great career, or a loving spouse. Not everyone is lucky enough to have that. So count your blessings and be grateful for what you have.
Forgiveness. It is not just an abstract term. It is one of the biggest virtues, but the hardest to practice. When you forgive the person who hurt or betrayed you, you are not being a coward or condoning the person's behavior. Instead you are being the bigger person here and are freeing yourself from anger and hatred. By forgiving the person, you are finally letting go of the past and paving the way for a happier future.
Once you are ready to let go of past resentment and bitterness, you will feel that your life is enhanced. You life and mind would be open to other more exciting possibilities and you will be able to lead a much happier and calmer life.