I have a recurring problem in MTT’s if anyone can shed some light.

An example from my most recent mid-stakes marathon online tournament.
I’ve played good poker and have worked from 10000 to 50000 stack size putting me in the top 5% of stacks.

I have ATs utg and raise 2.5bb. Sb shoves 10000 all-in. I immediately decide I’m going to fold, the Bb shoves 10000 all-in. I still want to fold but I’m blinded by the potential reward and call. Sb wins with AK. I then quickly lose another 10000 chips to a bluff that I thought I should call but didn’t.

Now I’m tilted, after a long time working my way to 50000 chips and not making any mistakes, to quickly losing 20000 by going against my instinct.

I quickly blow the rest of my stack and I’m out of the tournament.

This is a recurring theme for me. If I could maintain consistency I could do MUCH better in mtt’s, but when I make a silly mistake and dent my stack it sends me in tilt.

Any help would be appreciated. Many thanks.

Also, how do you deal with the regret of wasting such opportunities? These are the situations I have worked hard studying and practicing for, and when they arise I seem to make a move that goes against my better judgement and blow it.

this is known as "mistake tilt" one of my biggest flaws throughout my career, you need to identify the underlying flaw in your logic that causes you to tilt; ask yourself why do tilt when you make a mistake? why is this logical? why is this logic flawed? what is the correction to the flaw?

Mistakes are going to happen, the bigger mistake is allowing one to turn into more

I had a bad habit of beating the **** out of myself for years and I've recently been working very hard to resolve this flaw; your fundamentals can be flawless but if you're mental game is weak then It can cost you a lot of money

I tilt because it has taken so much work (time, patience, attention, care, calculation) to get to where I am, and so much of that is suddenly lost with one impulsive move. I don’t beat myself up but just have this horrible feeling of utter regret and disappointment, which is so overwhelming that I can’t continue with my normal game.

Perhaps I need to find a way to maintain a state of mind where impulsivity can’t enter.

The first step is to work on not making the big mistakes in the first place. I've used reminders on my phone, such as "Your only goal is to play THIS hand the best way possible." That's all that matters. You can also use perspective-shifting questions, like:

"What would I tell someone to do in this spot if they posted this hand on 2+2?" Or

"If a friend told me about this hand, what would I advise him/her?"

These questions help you step outside your own perspective and take a more objective view of the hand.

The second step is to get the mistake tilt under control. Take some deep breaths and step away from the table. Get back in the moment by reminding yourself that you can't control what's already happened, you can only control how you play the next hand. Don't think about the mistake you made or how many chips you used to have. Focus on how many chips you have now and the best way to play the stack you have. Focus on each hand as it comes and the rest will take care of itself.

Your goal here is to prevent your emotions from breaching the threshold which will then shut down your ability to think; you can have pre-loaded injecting logic statements like the poster above suggested; these can help you; look to recognize the early signs that you are getting emotional; if it is something that happens suddenly in just one hand then perhaps this is a variance issue

Thank you for your responses - they reinforce my own thoughts on the matter. Do you think there is any value in sitting out for a few hands after making a mistake and coming back to the game with the feeling of starting afresh? Therefore negating the psychological problems that follow after making a mistake?

your goal should be to prepare for these situations in advance and look to make small improvements, slow and steady progression; taking a break could help in short term but that's not a habit that you want to create

Absolutely. Until you learn to call down quickly and not let the mistakes tilt you, stepping away for a few min to call down is the best way to prevent further mistakes.