How to define my identity? Who am I?

There are actually 2 answers. One is how I view myself. The second is how the world describes me.

And I just view myself as a mundane, flat line.

For example, you learned Russian and you speak it, it does not seem any difficult or "amazing" to you when you scroll speech-heavy TikTok or YouTube. You hoard some data, build some userscripts, code a full stack website, use mostly cli in daily life, it doesn't seem anything abnormal either. You get used to yourself very quickly. If you learned a skill 1 year ago, you already do not consider it anything remarkable. It becomes a fact that you can code or you are bilingual or something that you simply register as utilitarian as "able to walk", not a clickbait video title.

Actually, if you are to constantly remember and remind yourself of every achievement they had in life with a long list of titles and awards, you will struggle to do almost anything new. If you aim to shock people with your achievements, you are not loyal to yourself anymore, and nothing kills motivation faster than that. All these pop science channels are permanently stuck in a very low level. Doing and moving forward requires dropping the ego. You are rarely "surprised" at yourself. For me, a healthy state for an individual is normalizing your skills or achievements and focusing on the current or the future.

If you try to find your achievements that you can boast to the world, it is usually different from what your actually consider special in your life. For example, having a TikTok channel can be very special, but it is a terrible thing to boast professionally (and even casually, most of the time). Also, you may consider getting a 65 on one very objectively very easy exam to be more impressive than getting a 95 on an objectively very difficult exam. For neurodivergent individuals, tying the shoelace may be more difficult than linear algebra, and not being sincere can be more difficult than speaking 4 languages.

You are always not objective about yourself. That's a highly unreliable narrator.

What about the world? As for the world, it's usually down to just dozens to 100 something people physically around me, or close family members and friends who regularly text and call me. The "reality" is merely perhaps less than 100 people. The rest is an illusion on the Internet.

This is a terribly unreliable narrator as well. First, what crowd are you in? If you are in a crowd with 90% extremely nerdy male, or 90% high earners, or 90% provincial people, the environment itself becomes heavily biased.

Furthermore, the environment gives very polarizing, changing feedback to neurodivergent individuals. This is inherently very unreliable as well.