Confused

“When I started developing breasts, I never ever felt comfortable with them. I still feel that way today. I just don't feel like they belong. I wear sport bras and sweaters to try to hide them.”
Not everyone who dislikes having breasts identifies as male. While it’s probably more common to as a female wish for larger breasts, there are females who are happy being female but would just prefer to have smaller breasts or even no breasts at all. Also while some trans men do intensely hate, dislike having breasts to the point that if they knew it wouldn’t kill or scar them or bar them from having sex reassignment surgery for the rest of their lives would just rip them right off their chest if they could, not all trans guys hate having them that much. A lot just depends on how dysphoric they are. Some trans guys are very body dysmorphic and others not so much.

“I've been wondering about what it would be like to be a boy. Am I one? Or am I female? Some days I feel more Feminine than other days. Some days I just don't care what I am. Some days I wish I was a boy.”
Wondering what it would be like to be a boy or wishing you had been born one is very different than knowing despite all physical evidence to the contrary you see when you look in the mirror you ARE a boy. All people at some point or another in their lives wonder what it would be like to be the opposite sex. Even trans people wonder that!

“Sometimes when people use feminine pronouns to address me, I will correct them in my head or think about who I am.”
This is pretty common among trans guys when they’re children, teenagers, but I don’t know any trans adults who still do that. While I do think silently, secretly correcting people schooling them in your mind about your true gender is highly indicative of some sort of gender dysphoria going on that doesn’t necessarily mean it is transsexualism going on.

“Sometimes I'll look at myself in the mirror and imagine myself with no breasts, and shorter hair.”
This is something a lot of non-gender dysphoric people have at one point or another done in their lives even if typically you don’t hear it spoken about openly much. It’s for example really common among teenagers of both sexes and later in life lesbians as well as heterosexual gender benders.

“Is this just a phase or something else? Does anyone else do this too?”
It very well might be a phase especially if you are young. However in general the younger you are when these feelings begin, the more they remain the same or intensify over time and the longer you have them the less likely it is to be a phase and more likely it is just an integral part of who you are. As for others yes plenty of people for a variety of reasons do these things.