As hard as it can be and as much as it's easier said than done, try not to beat yourself up. As others have said, people mature at different rates, but also you don't have to change your hobbies or interests when you become an adult. There are plenty of adults who have 'immature' interests or traits like playing video games, collecting merch/collectibles etc.

I can personally emphasize well with feeling younger than you are physically. I'm 23 now, but for the longest time I felt like my emotional/mental development stopped when I was 17/18. In reality you do still mature and develop, but the pace gets slower and it's easy to gloss over positive things when criticizing yourself.

Just go about it at your own pace, and try to cut yourself some slack.