First, watch this video about imposter syndrome. It's totally normal (and expected!) that for some patches of your career, you feel like not enough. These feelings often come at transition points in your career, e.g. a new job, promotion, or recognition.

One tactic I recommend is to better leverage 1:1s to get the detailed feedback and start to act on it. Here's the masterclass about 1:1s.

• You should definitely have a recurring 1:1 with your manager and talk about your feeling that you are not improving. Make sure this feeling is not just in your head, and others feel it too. (you may be surprised to see that your manager thinks you're totally fine!)
• Have 1:1s with other people on the team as well, e.g. your tech lead, peers on the team, and peers on adjacent teams. Be vulnerable with them and have discussions about your work and what you could do better. 1:1s are a time to be awkward!

I'd encourage you to be more structured in your reflection. When you say you don't feel like you're improving, are you saying that based on lines of code, experiments you've run, general vibes, or something else? When you have a metric or something you're tracking, you can start to actually measure it and focus on improving.

Finally, keep a brag journal to remind yourself of your wins!

“Spend each day trying to be a little wiser than you were when you woke up. Day by day, and at the end of the day-if you live long enough-like most people, you will get out of life what you deserve.”

―Charles T. Munger

First of all, since you joined only an year ago, don't be demotivated by not getting promoted. If there are others in the team who have been there longer, they have an advantage over you.

Secondly, looks like you already have some high-level feedback (" not assertive, opinionated, and take more time than usual to complete ambiguous tasks") It's time to dive deeper into it and figure out how you could improve on these. Ask specific questions to your manager like what you could have done better in a particular situation. Also, learn by observing other engineers. This could be just an issue of perception as well which is critical for promotion.

Some tips that might help you:

• Actively participate in meetings by asking good questions and suggesting solutions / improvements
• When you propose multiple solutions for a problem to the team, always go with a preferred solution (based on data of course). Be willing to change your approach later. But don't always depend on others to help you decide.
• For ambiguous problems, try to gather as much data as possible initially and come up with a solution. Don't wait until all the data is available because that might not happen. Make changes to your solution iteratively as more information becomes available
• Work on promoting / selling your work.

Hope this helps.