First, step ‘outside’ of yourself, so to speak. Imagine, without actually imagining or envisioning this, that you’re not there (that is, with the occurrence of the emotion and its effects). Don’t personalize the emotion. This is simply something you’re watching on a TV screen. Watch ‘it’ as if it were happening to someone else. In the courtroom, the judge often applies the ‘objective bystander’ test, that is, what would an ordinary man or woman think of the situation. However, with mindfulness, you don’t think---at least not in that sense---but you can be that objective bystander more often than you think.
Secondly, look at the emotion. Look at it. Really look at it. Notice. Observe. Feel it---fully---but remember it is just happening. It is not ‘yours’ unless you choose to identify with it---that is, ‘own’ it.
Thirdly---well, I just said it. Don’t identify with the emotion. Apply the ‘objective bystander’ test I referred to above. It really helps. Also, don’t analyze the emotion. Don’t judge it. If you do those things you have well and truly identified with the emotion.
Fourthly--and this is the hard one. Don’t do anything to ‘make’ the emotion go away. (That sounds counterintuitive, doesn't it?) Wait, there's more---don’t fight the emotion in any way. You know why---'whatever you resist, persists.' Right. You see, the more you fight an emotion, the more power you give it, and we need all the power we've got. It’s as simple as that, yet we take a long time to learn that. I often quote this line from J. Krishnamurti, ‘In the acknowledgement of what is there is the cessation of all conflict.’ They are very powerful words---and the spiritual principle behind those words is stronger still. Let it be. Yes, that’s right. Let the emotion be. You see, in order to let something go, you must first let it be. That’s bedrock. It’s metaphysical law. Stop fighting against the emotion. Accept it as it is. In time, the emotion will dissipate. And remember this, no emotion has any power to hurt or harm you unless you choose to identify with it. Now, when you fight against an emotion, you are well and truly identifying with it---you now own it, lock, stock and barrel.
Fifthly, be willing to get on with your life---despite the emotion. In 12-step recovery programs you hear a lot about willingness. Twelve-steppers say, for example, that if you are not now willing (to change, etc), then you’d better start praying for the willingness to be willing. Now, can you be willing---perfectly willing---to go on with your life while this emotion passes through you and in time disappears? Of couse, you can. (You know right now that would be the right thing to do.) The emotion will disappear, you know. Even people who are more often than not bitter and resentful aren’t always bitter and resentful, strange though that may seem. Emotions do come and go, and if we don’t hold on to them, they can disappear quite quickly indeed. So, make a decision to get on with your life. Then do so.