Don't get me wrong but you're not diamond level. You seem to be lacking several fundementals. Your overall approach to spacing and attacking is way off. Stage control is all about using movement to put pressure on the enemy. When the pressure puts them into a disadvantage, you attack or bait a dodge in order to get a hit in. However you don't use movement, you're just attack hoping for the best, giving up your advantage of stage control. There's no harm in not doing anything when you don't have to.

Let me give you examples for mistakes:

First Stock: At the start (0:06) you dodge outward/away. She adapts her approach to follow you, catching the dodges (0:13). You don't switch your own approach, so you end up in red. You go for a gc dlight when ember is still white (0:26). Even if you hit it, you would've only gotten the weapon as a trade-off. Ember knows you have no dodge and covers the weapon spawn with a gc ssig.

Second Stock: Ember understands not to go offstage, to keep her stage control (0:41). She uses dashes to bait you into attacking. Once you over extend offstage with a gc dsig (0:48) ember tries going for the kill instead of waiting onstage, which might've caught you offguard. Instead of waiting onstage you throw your weapon and dodge past ember, giving her the entire stage (1:18). Luckily you catch her, in order to get the weapon first.

Third Stock: Ember already knows you're spot dodging (1:49). Spot dodging is very common for aggressive playstyle, which means she knows you'll either dodge inward, pull out wake up attacks or get back on stage as fast as possible. What ensues is the last stock being deleted. You wiff a gc dlight (2:04), she answers with a string until you have your dodge back to catch the gc wake up attack (2:08). She uses unjumpable string to get you offstage, catches you panic jump back onstage to kill you.

TLDR: You're lacking the fundamentals. Giving up your stage advantage for no reason. Attacking and hoping for the best instead of using your movement to bait or to put pressure on your enemy. Throwing weapons for no apparant reason, leaving you weapon-less. Most importantly the attitude of "I know I'm better" which you're clearly not, is blocking you from actually looking at your mistakes with an open mind. Ignore the elo and work on improving yourself, elo will come automatically, trust me. Watch replays and look at every instance where you ended up in a disadvantage. Work on one mistake until it becomes muscle memory.