My advice would be to think one step ahead before any making any decisions in an MTT. Whatever action you take, you should have a reason for doing so and nothing should be done out of impulse. My taking the time to think about your actions will not only help you make the possible best decision, it keeps your mind on the game.
Taking your post for example, you snapped called a flop shove with a flush draw for more than half your stack. Now, did you really think about what you doing when you called or did you just see a flush draw? For the sake of this post, I'll just say you saw the flush draw and possibility of picking up a nice big pot got into your head. Maybe you got ahead of yourself and thought that if you won this pot, you'd be a commanding chips stack.
Now, if you had stepped back (so to say) and really broke down what has happening in the hand, you might've made a better decision. You may noticed things like:
1. You weren't getting the odds to call and chase a flush draw.
2. You were calling with 5 high spade draw, is that really a hand worthy enough of calling half your stack for. Your opponent can easily be semi-bluffing with a higher spade draw here.
3. To call cost you half your stack. If you lose, that's a huge chunk. If you fold on the flop, you give up very little and you're still sitting on a great stack.
4. The risk just isn't worth the reward here.
So, think about the actions you decide to do. There should always be a reason for your actions. Then think one step ahead. For example:
1. Limping in
-If you limp in and someone raises, are you going to call? If no, then don't limp.
-Is it a hand you could raise with? Then raise instead of limping.
-Are you calling a raise out of position? What will your action be if you miss? If you hit?
As for losing a ton of chips in hand, if you're still in the tournament you've still got a chance. Look at your stack compared to blinds. If you go from a 40K stack to a 10K stack and blinds are 25/50, even though you lost 75% of your stack, you're still sitting on 200 big blinds so you're still doing really well. Keep that in mind. What did you do before you build the big stack? How did you play? Unless you're running super well and winning every hand, your stack is going go up and down through out the tournament. Just keep on adjusting.
Here is small self-evaluation guide on MTTs
I wrote. There may be somethings in there that could help you.