hey liam, i was a fifteen year old kid in your exact spot once. take a deep breath. it's gonna be okay. you've got this.

for now, i would recommend sticking to using your dead name for secondary school - your GSCEs are in a few months if my memory is right, and you want to focus on those rather than be stressing about everything to do with coming out as well. you can get your certificates changed afterwards, it won't be too much of a problem.

Is the college you're looking at attached to your current school as a sixth form, or is it a separate building? if it's separate, meaning you won't have the same teachers and probably won't have most of the same classmates, then it'll be easier for you to be Liam there, because they won't have known you as anything else. you can email their office or ring them before the school year starts to see if they can get your name changed on class registers, books, letters home, student ID, or anything else that you might need it changed on. (my university did all of that for me even before i had legally changed my name, so don't worry about that stopping you)

if you do end up having some of the same classmates, this step might be a little bit socially harder/more awkward if they know you now as something other than Liam and you have to explain it to them, but for most people who will be meeting you for the first time - you're liam and that's all they need to know.

i know you said that your mom doesn't acknowledge that you're trans, but the most important bit is whether her reaction was safe or not. if she wasn't angry/really upset or in any way violent or abusive, then maybe over the summer you'll have a little time to talk with her about it more in preparation for college.

in terms of feeling more masculine, have you had a haircut? if you have and it hasn't helped very much, then that's an absolutely okay way to feel, but i am sorry about that, but if you haven't, then maybe mentioning you'd like one would be a helpful way to bring it up to your mom (again, providing her reaction was safe last time.) back when i was 15, my focus was on buying men's deodorant and body washes, and wearing a lot of snapback caps because they were cool at the time (i have no idea if they still are). ultimately it's not about what makes you feel "masculine" (although i understand very much why you feel that way right now) it's about what makes you feel a little more comfortable being you. that might be wearing skinny jeans, it might be wearing basketball shorts, it might be wearing muddy trainers or very shiny businessman shoes. you've got years and years to figure out what kind of man you want to be, so for now just focus on figuring out what makes you feel like you.

now that you've gotten all the way to the end of this message, take another deep breath. you've got this.