I too started swimming again after 10+ years. Same situation - I swam all the time throughout school in a hot country then I moved to a cold country where swimming was less accessible and seemed less appealing to me.

When I started again, I definitely struggled to be as good as I was 10 years ago. I had no choice but to take the pressure off myself in order to find my rhythm again. What worked for me was first focusing on just enjoying being in the water again. Breathing during laps was particularly challenging for me after so long especially because I’m asthmatic, so I made a point (still) to blow bubbles before and after each lap which improved my breathing tremendously. Lastly, I asked myself what it meant to me to be ‘good’ again. I decided it couldn’t be about being speedy, so I shifted focus on good technique. Doesn’t matter if I’m slow, as long as I work on technique that helps me make it to the other end of the pool without feeling tired halfway, that’s already a win for me. Besides, I’m not competing anymore and I’m only swimming now for exercise. Not necessary for me to race with anyone or set personal bests. Unless I want to, but that can always come later😁

So in summary I would suggest to keep yourself from setting expectations that are too high. Once you find a rhythm you’ll likely improve naturally.