What does "give a boost" mean?

How different "may not have gone" and "may have not gone"?

In the context of this clip there would be no difference between "may not have" and "may have not". They would both convey the idea that they did not experience the same thing as her. However "may not have " sounds more natural and is more commonly used. Ex: They may not have eaten the apples. = They may have not eaten the apples.

Can I use "that" instead of "what"?

"That" can not replace "what" here. Another way to say this sentence is, "Now I understand the feeling of being a loser."

What's "pull" mean here?

Here "pull" is a part of the phrasal verb "pull through," which means to get through a dangerous or difficult situation or time. Ex: He pulled through after losing his job and found a new one! Ex: The exams are going to be tough. We'll pull through this next semester.

Why do you put "the" in front of "move"? Should I always put "the" when I say this expression?

To "be on the move" is an informal idiomatic phrase. Because it is an idiom it is always used in exactly this form and structure. Its use is to express that someone is traveling, going somewhere or being physically active. Ex: I've been on the move all day. Ex: Are you on the move now, or are you still eating? Ex: The army is on the move at last.

What does "give a boost" mean?

To "give [someone] a boost" means to lift someone up a bit so they can reach something or somewhere. To "give a boost" can also mean to increase in energy or production due to an added factor. Ex: Can you give me a boost? I can't reach my book on the shelf. Ex: Boost me up. I'm almost over the wall. Ex: The investment gave a boost to the company's overall production rate.