What is "shoulda"?

What's "doomed" mean and when's it used?

Here, "doomed" means to have an unfortunate and inescapable outcome. It's destined to fail. It's used to describe things that will end badly no matter what. Ex: We're doomed if our parents catch us sneaking out. Ex: The plan was doomed to fail. Ex: The dog was doomed to the streets if we didn't take it in.

Is it common to use present tense instead of past tense when talking about childhood?

It depends on the context! For example, the man's childhood bed still exists, and in fact, both he and Pikachu are looking at it this very moment. Thus, it exists in the present moment, making the use of the present tense here appropriate. If they were just reminiscing about his childhood and talking about things he used to have, then the simple past tense would be best used here. Ex: Wow, I can't believe that your childhood room is pink! It's so princessy in here. => hint that the speaker is inside the room right now. Ex: Wow, your childhood room was pink? You must have been a girly girl. => just speaking about the past here, so the past tense is fine.

Why would Taylor Swift mention "Love" in this context?

Here, "love" is a way to show who the letter was written by. It's instead of "from" or "regards." It's more affectionate. So I wouldn't use this formally with friends and acquaintances. I'd say it's common with loved ones, close friends, and family. She puts "love" here since it is written for her children. Ex: Happy birthday. Lots of love, Mom. Ex: I'm excited for your visit! Love, Henry.

I suppose the word "level" refers to a height of something. But, would there be other alternatives to replace the word "level"?

In this case, you could use the word "amount" by saying "the amount of water had increased" or simply "there was more water". However, it is most common to measure large amounts of water using the word "level". This term works for baths, the sea, dams, lakes, and rivers. This is because it's usually in reference to the ground or a lower surface. Ex: The sea level had risen since that morning. They couldn't cross the beach to get back. Ex: There had been a drought for about a year now. The water level at the dam hadn't risen enough. Ex: After the rain, the river had more water flowing in it.

What is "ya"?

"ya" is a replacement for "you." "Ya" is actually how a lot of people pronounce "you" in a casual conversation. Ex: Can ya give me an example? = Can you give me an example?