What can I say/do when I am about to explode (for any possible reason) and need to stop or take a break on the spot even if I am in the middle of something - due to feeling overwhelmed, stressed, extremely frustrated or any other high emotional-aggressive outburst at work - without putting my job at risk?
It does depend what you're in the middle of.
If you're just sitting working alone, you can probably just take a break right then and there without saying much / anything.
Ask for a break
In some situations, it could be as simple as:
Do you mind if we take a few minute break to regroup and get some coffee or a snack?
Or something a little more assertive:
I think now would be a good time to stop for a quick break to regroup and get some coffee or a snack. What do you think?
Ask for a bathroom break
In other situations, you can probably get away with something like:
Do you mind if I take a quick bathroom break.
(and actually go to the bathroom - you don't want to be a liar - you could even just splash some water on your face or something)
This is probably more for a one-on-one setting with your senior, in case you don't feel comfortable with the first option.
No respectful person will question your bathroom habits, unless it's seriously getting in the way of your work, or it's unbelievably often, even for someone with valid medical reasons.
Under other circumstances, you could perhaps try planning your meetings with a break or two in the middle, assuming this builds up rather than just coming on suddenly.
Discuss it with your boss
This still may not cover all situations.
If other situations are common, you should perhaps consider raising the issue with your boss - as long as you choose your words carefully, and he/she is understanding, it shouldn't go badly, and you should be able to work something out.
Extremely overwhelmed / stressed are probably the words you want to use - I would steer clear of "explode", "aggressive" and probably "frustrated" - those might across as you wanting to hurt someone, which is definitely not an image you want to portray.
Note that you should probably deal with the cause of needing these breaks - perhaps you're unhappy at work, perhaps there's some underlying reason, either way, it's likely to get worse. If you can't identify the cause yourself, you should perhaps consider going to see a psychologist / psychiatrist.
Breathe. For as many breath cycles as you can afford in time (even one can help - personally I aim for at least three cycles, at most a few minutes worth), move your focus away from your work and just notice your breath coming into your body and then leaving your body. Usually by the third breath, my shoulders will let go and my mind will feel begin to feel clearer.
This is an idea promoted by Janice Marturano (former VP of General Mills) in her book Finding the Space to Lead: A Practical Guide to Mindful Leadership (2014). She recommends ten minutes of breathing (I have never made it that long):
Bring your attention to the sensations you notice as the breath enters and leaves the body. Whether you feel the breath most strongly in the nostrils, in the chest, or deep in the belly, aim and sustain your attention on the sensations of the entire in-breath and the entire out-breath in that area of the body.
When you notice your mind has drifted off - for example, running through your to-do list, rehearsing what you are going to say to someone, or playing a little movie in your head - redirect your attention to the sensations of your breath. There is no need to change your normal breath and no need for deep breathing exercises. Just notice the sensations that are already available to notice. There is nothing to change, control, or fix. Just be aware of the way the body experiences each in-breath and each out-breath.
If you are really upset and are physically affected by adrenaline, you may need to move as well. A few minutes walk, thinking about the feel of your feet on the ground instead of anything work-related, can help relieve the flight/fight feelings. Personally, I find that if I can get out of the building, even for one minute, into fresh air and sunlight, it makes a huge difference.
The trick in breathing or walking, though, is to stop thinking about work, to stop thinking at all!