Gunnysacking happens when we take irritations, slights or offenses from others and toss them in our gunnysack rather than either dealing with them by a candid conversation or forgiving them. One cannot carry around a heavy gunnysack forever without the weight of it affecting us. When the gunnysack gets full enough of unresolved issues it is likely to explode with an eruption of emotion that we regret afterwards.
It is not always offenses that cause us to erupt. It can also be irritations to us from the quirks of others. All of us have quirks of personality which can irritate others. When we allow those quirks to fester as irritations to us they also can cause us to erupt in unhealthy ways of we have gunnysacked them. Often this happens with extended family when someones attitudes, biases, actions or words irritate us to the core but there is nothing we can do about it.
How do we prevent gunnysack eruptions? First, always be aware of the danger of gunnysacking when we are with people that irritate or who have slighted us. Remember that tossing those irritations, slights and offenses into our gunnysack is going to hurt us in the long run.
Second, keep short accounts. Most issues that we take as slights or offenses do not come from ill motives and judging motives is the worst thing we can do as we are usually wrong in our judgement. If needed, have a conversation where you can clarify the issue: "This is how it felt to me when you said or did such and such and I am sure you did not intend for me to feel that way, so can you help me understand?" Much better to get the issue on the table than to toss it in your gunnysack.
The harder thing to deal with are irritations. Sometimes it is good to tell people that when they do such and such it is irritating to you. Other times it is best to simply give them space and even to limit your exposure to them if having a conversation about the behaviors will be counter productive. There are people in my life that push buttons in me and the best way to deal with it is self awareness and limiting my exposure to them.
However we deal with issues that irritate or cause offenses, the one thing we don't want to do is throw them in our gunnysack and carry them around. The weight is unhealthy for us and eventually may cause an unhealthy response on our part.