It's awesome that you acknowledge that your reactionary behavior can be toxic. I feel that is one of the hardest things to recognize in oneself as a player. So good for you for wanting to change!
I actually heard this advice from someone on my team but if you run across someone being toxic, you just need to ask them one question: "Hey, are you feeling okay?" I feel this question not only gives you a chance to calm down when you want to react in a toxic way, but also this question isn't belittling to the toxic player and is addressing their behavior and not attacking them as a person. This question gives other "reactionary toxic" players a chance to check themselves and react. I was surprised to hear that a player once responded with: "I'm sorry I'm not really good at this game and it's really bothering me." After that, the other team members were helping them with advice and tips and the player stopped being toxic. You start to see that some players that come off as toxic are really just upset with themselves or other random things and the game just triggered them.
If that doesn't help you, the other advice in the comments like breathing, muting others, and just communicating information can help too. :)
It's not going to be an overnight change, but be patient with yourself (and others). I hope this helps, good luck!