How do I keep myself from overreacting?

Distraction. Distract yourself as much as you possibly can while angry to let yourself cool down initially to whatever it is that upset you.

Pause before you react to any given situation. Try starting a daily mindfulness meditation practice. I find that I can more easily become aware of my emotions now that I've been practicing mindfulness for a while. I'm able to think to myself "I'm starting to feel upset now. I'm aware that I'm becoming agitated" and just having that moment of awareness gives me the ability to think about how I want to respond to that emotion. Usually I'm able to take some deep breaths and ground myself, sometimes I use self compassion and say "I'm having a hard time right now, everyone has a hard time sometimes, I can get through this, it's okay to be upset." etc. until I get through the big feelings.

Remove yourself from the situation and give yourself a minute to think about it, think about what has happened, and analyse it, is it really that bad? Try to see things from another point of view

Every time you feel panic, force yourself to stand still and not do anything. Repeat the thing that made you panic in your head and try to analyse it. "How should I interprent this?"

Sometimes, certain things are more difficult for us than they are for other people. Especially if you have a personal history of dealing with difficult problems. When the body and mind experience traumatic situations or even emotionally difficult ones continuously, it becomes very difficult to handle even small things. Relaxation, meditation, and listening to your body are the best ways to re-set your nervous system so that you do not feel so much anxiety over small things. Consider getting help from a therapist or counsellor who can work with you to overcome your anxiety. Anxiety is scary and difficult, but it can be fixed with the right resources.

We overreact when we react instead of responding. React is in-the-moment and taps into our other-than-conscious self. Recognize the triggers and situations, and if you can't avoid/minimize the situations; train yourself to breathe, take some time, as little as a few seconds or... longer. Then formulate the proper response, and account that possibly "do nothing / say nothing" can be the most appropriate response at times; and while do-nothing might not be optimal, it might be better then overreacting. Keep practicing responding in lieu of reacting, and little by little you'll improve.

Take a deep breath, mentally step away from the situation and think it through before you react. I know it might seem difficult to do but with some practice it gets easier

I start by slowing down my breathing. Once I do that, I try to look at the situation somewhat objectively and ask myself what would be possible next steps, what caused the situation, and how I can learn from it. All before giving a response to the situation.

Try to think about calm thoughts. Once you have calmed down a little, if it's another person's news that has you overreacting, try to imagine this from their perspective.

Take a deep breath and ask yourself if it's worth reacting about. Be realistic with your answers. Maybe even take yourself out of the situation for a second and think if it makes sense to react how you want to.