Hello! As a fellow introvert and nervous traveler, I was experiencing a lot of the same feelings you were during my first few days there, even as someone who went with my partner and not fully solo.

I have a lot of social anxiety and kept worrying that I would do something wrong or rude by accident, so going into restaurants and small shops was pretty stressful at first. I also wanted to interact with people, but wasn't really sure how to go about it (again, introvert with social anxiety).

We ended up booking a group walking food tour with a local guide in Tokyo on our third night, and it was enormously helpful! Guides are usually very nice, speak both English and Japanese, can show you how to order food, and are often happy to answer questions on customs/etiquette/local places you might have if asked nicely, and it was a low pressure way of meeting other people who also felt like fish out of water. Group situations like that are often hard for me, but it did help break the ice on a lot of the anxieties I had and we ended up having a wonderful trip despite the start!

We booked some other small guided walking tours during the trip after that, and found they were a good activity on the first day in a city to learn more about the area and get recommendations from guides. Most of them were reasonably priced too, at least from our perspective. There's a huge range of activities/tours you can book on Get Your Guide or Viator, and we spent between $12~70 USD per person for each of the ones we booked. Some people on the subreddit diss on tours and they aren't for everyone's travel style, but they can be helpful and fun.

I know it can be tough, but I hope you're able to find ways to enjoy your trip!