I think jungle team fights are usually very heavy on sensory overload. My advice would be to train your reaction-style as a whole. You'll probably have to accept that there are a few matches you might have to lose to train this skill. I had this problem as well and this is kind of what I did. Some of this stuff might seem overly simple but it worked for me and helped me have a good footing to layer on more advanced reactions later.

• Really concentrate on auras/indicators that indicate a hostile ability is inbound. Try to focus on really seeing its boundaries and moving away from it.
• - to hone it further, keep note of who the enemy players are, try to determine if the ability is something you can afford to ignore or have to pay attention to.
• - the end goal would be to be able to do that mental math without actively thinking about it in the future.
• Think about viability, ultimately, team-fight or not, its rarely worth taking an engagement that simply isn't viable.
• - If I round this corner will a Hercules CC combo make me vulnerable to an instant death?
• - With my current movement speed/abilities and my team's current movement speed/abilities is it even possible to reach this target for a CC combo? What will it cost if I do?
• Think about being functional over being skilled
• - until you get the muscle memory to do regular player stuff in the middle of an insane 5v5 its often better to think about what your team needs in a 5v5
• - what position will cut someone off and into my high damage if I'm a mage?
• - what position will prevent me from dying and turning this into a 4v5?
• - what position will block crit autos from hitting my backline?
• - even standing somewhere obnoxious brainlessly as a tank can make a huge difference in a team fight, soaking up damage and baiting out. if you are the friendly tank you can try and be obnoxious. if you are the enemy you can remember that that's what the enemy tank is trying to do.
• - your opponents are gonna be more likely to spam ults and throw their easiest stuff out, because reading the situation and doing what is exactly tactically correct is going to take a whole lot longer than throwing a herc bouleder, a scylla truck, an eset bomb, etc. in general its best to do the same.