Sorry to hear the internship didn't go as well as possible. Here are a variety of thoughts.
First, the only way to grow is to:
1. Know what you need to grow
2. Put work into growing
3. Learn from the right people how best to grow, or figure it out yourself.
Knowing what you need to grow is about asking for feedback effectively and internalizing that feedback. It sounds like you have a fairly good idea what can improve. You now need to dedicate effort to growing. The open source projects were a good example of trying, but I don't know how much time you put into it. You need to put a lot of time into these things to become noticeably better; otherwise, the improvements are too small to notice.
The best people to grow from are usually directly from your workplace. Are you still at the internship? If so, seize the opportunity to find one or two senior engineers, or you manager, to ask their advice on how best to grow. They'll also be able to give concrete feedback on your PRs so that you can improve specific things.
Most open source projects have maintainers that never get back to you. I had a 4-year PR open on Firebase which the maintainer just punted two weeks ago because, in the past four years, they moved past the feature I helped them fix. I think it's hard to get traction on those projects. Much better to build something with a strong peer of yours and learn together. Or do https://adventofcode.com. Do every year, every problem. You will get much faster as you learn which architectures work and which ones don't. Do not use any AI at all to help you code those solutions. There's no substitute for generating a bunch of code by hand in order to improve how you code.
If you're not in the internship anymore, get feedback much sooner next time during the internship so that you can make adjustments quickly. I recommend asking for feedback weekly for the first month, and then maybe every other week.
There's not an easy way to get better at these things than to put real time into it, and to learn from the best people. Why don't you work on this past year's Advent of Code and LMK how far you get in a week or so?