Can't concentrate more than a few seconds

I did samatha and metta meditation for like 4 years, without a time schedule, just when I have time. Now, my problem is that I can't concentrate even for a few seconds. Old memories come, old mistakes come up, and I can't stay still, can't stay sit. I constantly think about past decisions and mistakes. I can't even think clearly. I don't have a personal meditation teacher. I learn everything from books, YouTube, and the internet. Please give me some directions. 🙏