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It is hard to offer concrete suggestions or even precisely guess what's happening without more information about your practice and your background; that said, it looks like your practice is sincere, and it is working. Often when the practice begins to bite, irritability and impatience may increase - it is a learning moment, and a teacher. Find the three characteristics of impermanence, non-self and suffering in each of those moments of irritability, dullness and anger.
Without getting into too many technical details, I'd say, think of it as the next level in a game, your opponents are tougher, and that is what makes it more interesting.
The most important thing is not to abandon practice at this point, but to continue, this is exactly when you need faith and perseverance.
I assume you practice insight meditation (also called Vipassana), if you do, it is not about indulging in the pleasant moments, or treating ourselves to some peace and calm - it is about understanding reality - which means one encounters the good and bad mind moments, and learns to remain equanimous.
Everything is impermanent - even the good and calm times in meditation will go away, and be replaced with difficult times - this is the moment of wisdom, that reveals the suffering in being attached to any feeling.
You might want to take a look at another question where I've offered more advanced help - Working with cycles in meditation . Stop reading it if it gets confusing.
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When practicing insight meditation it can happen that latent defilements are being stirred up and surfaced. It can happen that a very calm person suddently experiences strong feelings of anger. One is "cleaning out the closet" in a way and stuff from the bottom can come to the surface. Do not worry too much about where that anger is coming from or what's the reason for it is.
The important thing is to work with it. To try to understand it and learn from it. Turn it into a foundation for cultivation of insights.
Anger is a variation of the second hindrance, i.e. Ill-will. A way to deal with this is to turn the feeling of anger into an object of observation. Try to see the 3 signs of existence in order to understand that this feeling of anger do not belong to anyone. The anger is not a Self, it is not you and it is not yours. It's an impersonal phenomena that has arisen and will cease to exist again.
It might be very intense but it's impermanent so sooner or later it will burn out it's fuel. By observing it and not reacting to it one is robbing it of it's fuel. If one interact with it or has aversion towards it one will provide it with further fuel to burn thereby making it last longer and burn hotter.
Another approach is to do metta meditation and thereby replacing the anger with loving-kindness.
Lastly, here is a video called "Monk Radio: Dealing With Anger in Meditation" by Ven. Yuttadhammo.
Let me know if you have any questions to what i wrote and i will try to answer. May you be well and happy.