Can someone help me understand why this position is winning for black by so much?

I suggest that you take the time to work through the lines with engine assistance. You will see that black's advantage isn't as large as you originally think it is.

I should follow my own advice. Black is crashing through.

Can you explain this? I tried to look at it again but I still don't see the advantage. Obviously im still 900 rated so I shouldn't be able to see everything but I still want to learn.

Black has a winning attack that forces white to sacrifice material and position to prevent mate.

How did I not think of that! Thank you so much.

17. a3??

Why did you play a3? Why not King to b1??

the pawn structure in front of whites king is completely shattered and there's no defense, that's why it's winning.

Yep.

Just look at the moves suggested by the analysis. The only possible defense for White is c3, and in a few moves he loses the Queen.

(Actually the game was already over, because Black had mate in 12 moves. Try Bishop h6 after White plays c3 and the engine will show -M11 at the right depth)

Missdanielcb is correct. Black has a mating attack.

Everyone is correct of course, following the analysis will tell you why concretely here black is winning. What the analysis will not explain in a straightforward way (it can indirectly if you look through a lot of games and try to make your own conclusions) is what went wrong for white, and what is the truth behind this position.

For instance, 5. Bxf6 makes sense here, but one would expect white to take the pawn with 6.Nxd5 attacking black's bishop in the process, and after it moves to g7, c3 makes sense because it kind of restricts black's bishop. In this position white is a pawn up, but black has a bishop pair, and the position might open up, and black might have some compensation for the pawn.

But if white plays 5. Bxf6, and do not take the pawn, then perhaps he shouldn't have taken the knight, though it might still be ok if he manages to keep the black's bishop at bay with his pawns. Nevertheless, even if it is ok, it is more difficult to play it like that, and if white doesn't have this in mind beforehand, he shouldn't trade a bishop for the knight here.

In essence when one thinks about trading bishop for the knight, he should have some good reason for it, or he should at least have some plan afterwards. This is not crucial at this level, but still, it will not hurt either.

This is only an example of the things that you will not be able to understand directly from the analysis, so how can you learn it? By analyzing your games, plus by getting to know some concepts by passive learning as well (books, courses, videos, etc.). Sometimes you will be able to draw right conclusions from analysis, but only if you do it in depth, not in 5 minutes.

Anyway, I will not look through everything, in order to not spend a few hours on why every little move is an inaccuracy.

Let's rewind to move 15. Black is trying to attack. It is a reasonable plan, but the flaw is, that white must help black in order for this plan to work. White ignores the attack completely, and here he couldn't do it, because black wants to kick the knight and take on a2, after which he is much better and will have an easier game.

After 0-0-0, in many cases Kb1/Kb8 is a good move because it secures the pawn (and in some other cases it moves the king from an open file - which is not the case here). So Kb1, and after b4, and after the knight moves, king defends a2, and black doesn't have the attack. His queen blocks a7 pawn, and black is not ready to play c5 and open up the file because d pawn hangs. White will have the time to play h4-h5 and has an attack of his own. So if after b4, white plays Kb1, white is better.

In the game, after 16. Ne2, black should take the pawn on a2, one would thing this was the reason to play b5-b4. After 16. ...Rb8, Kb1 and black doesn't really have much. b3 doesn't do anything except that it looses the pawn. But instead, white helps black.

17. a3 is suicide and the move that defined this game. If you should understand anything from this game, it is the next few sentences. a3 opens lines in front of white's king. This is not what white should do. It weakens the king and in a few moves it is over.

Sure, it is not so easy, the straightforward bxa3 is not what black should do here concretely (though it is tempting) because white can take with the queen and either get queens of the board or make black lose time by retreating his queen, after which the game continues. 
But then white plays 18. bxa3 and the game is over with correct play. Sure, you need to see concretely how, but without calculation, you need to see how weak white king is. After bxa3, white gives black an open b file, in the vicinity of his king, and that combined with the fact that black has many pieces aiming around white's king makes this position hopeless for white. Queen, rook, and bishop, they all make white's king very weak here.

That is the reason why is this so winning, and this is what needs to be understood even if you do not see the concrete tactic.

In any case, always do your best to analyze your games to the best of your abilities. Little by little you will get better at it, especially if you get to learn some concepts like: pawn structure, material, activity, king safety, space, concept of time etc...

I hope this rambling will be at least a bit useful. In any case, best of luck.

Bh6+ is a pretty strong threat. To me it does not seem that your opponent "blundered" mate, you did not really give him a chance to escape.

I was black in this position

Thank you so much for the detailed explanation!!

Yes. I should have said 'blundered mate in one'.

You're welcome.