That's how you do it.

When somebody you've loved and served with all your soul leaves you, it's okay to be mad. It's normal to be angry for a time.

You should give yourself the chance to feel hate and resentment. Emotions that are deemed negative can always turn out to be your means to move forward.  It serves as a block for you to look back and start remembering things that belong in the past; it stops your tears from falling again and again. The world dictates that you should learn to forgive to set yourself free -- that is true...but that part comes only AFTER you've satisfied yourself of your right to shout out all the anger in your heart. You can't possibly feel so much pain and rejection and suddenly jump to forgiving when you know you deserve justice.  

If they leave, let them. Never beg. Never stand in front of  a door that has already been shut and locked to keep you out. And if you think of retribution by getting back at the people who've made you feel like shit, STOP. Giving yourself 'justified justice' does not mean stooping down and becoming too cheap. Let yourself be immersed in silent anger and let that feeling turn into unbreakable PRIDE. 

Love is just an emotion. You can turn things around and make it work to your advantage. Keep your heart on a tight leash for a while and let your goddamn brain work for once. Think of it this way: If you yearn to breathe fresh air in the midst of all the smoke, try to suck it all in first. Let the stench linger for a while. The more you expose yourself to intoxicating fumes, the more your body becomes immune. That ability to withstand everything becomes your growing armor. 

It's only in the beginning that people will forget you; it's only in the first stages of letting go that they'll treat you as non-existent. But so long as you've planted the best memories in their minds, trust me, even the coldest of hearts turn warm in time. In the process of letting anger overcome you, help yourself regain what you may have lost. Look at your image in the mirror and convince yourself that you are worth every goddamn respect. That overwhelming feeling of resentment will mold you into someone tougher; the feeling releases strength you have hidden in your system. By the time people realize what they lost, you'll be swimming in what you've gained. 

Forgiveness will come at the proper time. Don't force it. Remember that you cannot completely forgive when you have a broken soul. Premature forgiveness only satisfies THEM and leaves you drowning in self-pity. Forcing yourself too early to forgive and forget is like giving them the privilege of washing their dirty hands and get away scot-free. In real life, you know that is not what you want. 

Pick up the pieces first and mend your wounds. If you have to use pride and anger as adhesives, DO IT. Let time naturally sweep away all the negative feelings in your heart. It will happen without you knowing it. When it finally heals, you won't feel anger anymore. What's left is a very strong heart -- one that is willing to go to battle with a tough armor; one that is ready to LOVE again.  

This is not called bitterness. This is just how you deal with it in real life.