I have been a fool

Possession - Section 8 and Section 21

I have a tenant who occasionally ran into arrears, but he lost his job at the start of Covid and I didn't press him. We were on good terms and he always promised that as soon as he found employment he would begin to pay. He helps to support his estranged wife and children. He took another, low paying job, and began making payments. Our last discussion resulted in him making two payments and saying that he was taking a second, seasonal job. However, his arrears started creeping up in December and he now owes £4,000. I am not a professional landlord and can't quite understand which form(s) I need. He had an assured 6 month tenancy originally which rolled over to a periodic tenancy. No new agreement was issued. I am now struggling to pay the mortgage on the property and my mental health is suffering. He doesn't respond to messages and doesn't keep appointments. I've told him that I'm driving down on Friday to inspect all the flats. Any advice would be welcome, but keep it simple please.