Hey Stuck AF!
First of all, congratulations,
You have done a great job defining the things that you do not like in your life,
and are taking some action to change them (like posting here).
And let me tell you, I am excited for you, you have still so many things to learn,
and are starting your self development journey at a great age,
some people wait until they hit a major crisis in their 30´s to start making changes.
So, where to start? right where you are,
You already mentioned some of the things you do not want,
let´s think about them as ´the shadow´of what you do want.
What I would suggest you to do is define what it is that is casting this shadow.
For instance, you said:
I have self esteem issues -> is the shadow of being confident.
I am underweight -> is the shadow of being physically fit.
And this things (being confident, being fit, etc) are inside you, that´s why you can see their shadow
and that´s why they bother you, because something inside you is telling you ´what I am seeing is not the real me´.
Nothing has changed yet, but now instead of seeing problems (shadows) you are now seeing GOALS.
The reason why videogames are so addictive is because they have clear defined goals,
that you reach little by little, mission by mission, race by race, and challenges are increasingly hard
it is common that the first missions in a game are really easy, but you get some gear or you build some skills,
and as you progress the challenge increases.
It is this balance between the level of the challenge and your level of skill that keeps you hooked.
This is known in psychology as the state of flow. And as you know is addictive.
Imagine what would happen if you could apply this principle to your self development?
I am sure you would feel motivated to do things that will get you to your GOALS.
So as I said it is the balance between challenge and skill,
If the challenge is too big, you are going to feel frustrated, and not very motivated to try/play the game
if the challenge is too small, (ie it is too easy) you are going to feel bored.
Imagine if the whole game consisted only in beating the last (hardest) mission without having chance to build your skills?
how many times would you play it? 3 ? 5? I don´t know but I am sure you would eventually give up after being killed again and again without having chance to make any progress.
And what if from the beginning of the game you could beat all the enemies without any effort?
May be fun for a few minutes, but my guess is you would eventually get bored, quit the game and not pay it again.
Same thing happens in your life. (I have no motivation to start getting of my ass and fix the problems,I just sit in my room and days go by )
You need a GOAL and a set of smaller goals that will build your skills to the point where you can beat the big challenge.
For instance lets say you are going to play BEING PHYSICALLY FIT.
But being physically fit is somewhat vague, and as in any good game we need a CLEAR GOAL,
let´s say the goal is to weight X pounds (of muscle, not fat of course) or being able to run 300meters in under a minute or whatever goal you would like to achieve, (make sure you write this thing down and and keep track of it, as you would in a video game )
this has to be challenging, we are talking big boss challenging, and then divide it in smaller missions, that are still challenging but somewhat easier to achieve. Like the different levels on the game, and give yourself a reward every time you beat a mission.
That way you are going to be engaged and motivated.
The good thing is you already know the process, just in a different context.
Man! those are many words, hope they make any sense and help you.