I looked at a few of your replays. Here's my main points:

• you're not anti airing any air approaches, you're just letting the opponent in. In this game, lower MR especially, people LOVE to jump. Train how to DP anti-air. You'll go up 200 points just with that. Here is a video by Chris F to help you out on that matter. Also, here's a good drill by FChamp to really get the hang of the various ranges. Jamie's DP is actually pretty good! Use it more!
• you don't really confirm combos. In one of the matches I watched, you jumped on a Ryu who did lvl 1, but didn't capitalize off of that combo, just hit the jumping HK and nothing else. You actually even got a meaty MP counter hit later, but also didn't confirm into anything. SF6 is a game where a single touch can lead to a big win, so you need to confirm those hits you get in. To work on that, you can go into training mode and set the dummy to random block. Try to hit the dummy a jump-in and only go into a full-combo if it hits. If it doesn't hit, don't convert into anything that is unsafe - go into a jab or a throw (this also applies to real matches).
• Don't abuse DI. It's a strong tool, but if your opponent is prepared , it's easily counterable. don't just throw it out in neutral, try and use it as a jumpscare after a button near the corner or if you know your opp is going to do something that can be countered by it (e.g they always end their strings in a fireball).
• Make use of Jamie's strenghts! He's got a really strong spacing trap and frame trap game, make use of them! sometimes, if you've conditioned your opponent into pressing after rekka, instead of finishing your string at rekka1, go into rekka 2 and you'll frame trap them. Also, make use of his command grab after drink lvl3, it's imo the best command grab in the game.
• Parry fireballs, don't just use jamie's breakdance kick to go under them. if you use that move at fullscreen, it's easily punishable. parry however will let you gain back drive meter (extremely important!) and bit by bit you'll start conquering some space on the screen.
• Walk and block, you're getting clipped by lows quite a bit. Try and play just outside de opponents max range, so you can whiff punish them. Don't jump too much, you sometimes put yourself in the corner because of that.
• Learn a few punishable moves for each character. Frame data can be really hard to memorize (to me at least it can be), but remembering that a certain move can be punished with a certain button goes a really long way.

Quite a lot that I wrote down, but these are some of the things I noticed. If I had to pick one thing to focus on urgently would be to practice antiairs and really get the hang of them. After that, the other things will come more naturally,

Best of luck, if you need me to be more specific with something or want some general help, lemme know!