Why is the word "about" being used here? Is it ok to use "that" instead?

What does "course" and "so" mean here? Does "so" mean the same as "kind of"?

In this context, "course" means "period" or "length of time." Thus, we can understand "over the course of a year and a half" to mean that she was occupied with her album "over the period of a year and a half." "So" here means "thus" or "therefore," and she has used this word to express why her album is meaningful to her. Ex: I worked on my thesis over the course of four years. Ex: It's my most significant work, so it's quite meaningful to me.

What does "allergy" mean?

An "allergy" is your immune system's response to a substance - it causes an abnormal reaction. People have different severities of allergies and may react differently. Some symptoms include rashes, trouble breathing, gut reactions, and more. Common allergies include dust, pollen, peanuts, eggs, and bee stings. Ex: We didn't know the food had peanut sauce in it and had to rush Sarah to the hospital. Ex: I'm slightly allergic to dog fur, but I don't mind being near them. I just sneeze sometimes.

What does "stay civil" mean here?

To "stay/remain civil" means that the state of things remains polite, calm, or cultured. We often use this expression to de-escalate a situation where impolite language or violence may be used. When things are no longer civil, they are hectic, rushed, hurried, or even violent. The speaker is stating that with proper time management, all tasks get completed on time and everything stays calm and unrushed. He has used this expression to add a bit of humor to his speech. Ex: Stop fighting! Let's keep things civil. Ex: No matter what happens, we need to respect each other and stay civil.

What does "when" refer to here?

"When" in this context is a conjunction that is used to present an alternative or a different situation or option. Ex: Why pay for lunch when you can visit me at my restaurant and eat for free?

Why is the word "about" being used here? Is it ok to use "that" instead?

In this case, you can substitute "about" with "for" to keep the meaning of the sentence. If you wanted to use "that," you would have to tweak the sentence a bit, like "... I feel really selfish that I'm the only one eating it" to make sure the grammar is correct. Ex: I feel really guilty about showing up late today. Ex: I feel really guilty for showing up late today. Ex: I feel really guilty that I showed up late today.