I ghosted future employer after receiving offer letter

I am in an assistant role in organisation I. I had been offered a job after I interviewed on 8th April 2024. My notice period is 90 days(India). I discussed with my manager on 16th April for my resignation but didn’t send mail as he asked me to wait till his boss came in office on 30th. He tried to influence me emotionally to wait for 6 months as he was going through a crisis and his wife had a major surgery recently. Meanwhile future employer let’s say N) was mounting pressure to get early relieving as asked to my potential joining date. I gave buffer of 1 week and told on call my joining would be 23rd July, thinking I could manage them for 1 week. But I asked them to be flexible with joining date on the mail I sent to them.

When superboss came back, the next day they promoted me(same work, just change of role name) and gave 15% hike letter, while I had repeatedly told by manager earlier not to promote me. Now as I was under pressure I didn’t want to resign the same day as the promotion mail was sent to whole office. I took two more weeks to finally muster the courage to send resignation mail on 16th May.

After that I got to know the true behaviour of my manager. He is constantly coming up and asking me every hour where I was going and making jokes aimed at me but not directly to my face. I didn’t have the courage to ask for buyout or early relieving because I know my managet would object and wouldn’t provide it because whole of lab depends on me.

Now because of late resignation my notice period got shifted later by one month and the replacement they found for me took 15 days leave as she had to undergo surgery as well. Because of this my manager constantly harassed me to extendy notice period by another 15 days and said he will tell the HR to do that. Which again made by final relieving date to 30th August.

Because of this I thought the job offer from N would be gone and I foolishly ghosted their attempts to reach out to me thinking the opportunity is lost. But they called me with different number yesterday and asked what was going on? So I made and excuse and told them I would call them later as I was in a meeting. But I don’t have the courage to explain the whole situation to them because of the fear they wouldn’t understand.

I know what I did was very wrong. But what would be the best way to respond to N as a human and not in a give me the job kinda way. I thought they must have a backup plan and would go with other candidate.

This was my first job change experience as I joined this MNC straight out of college and have been here for 7 years. I don’t want to abscond because I need the experience letter and background check verification.