Her Grammy and Brit Award-winning album has sold over 30m copies worldwide since its release in 2011.
Speaking to Graham Norton for Adele At The BBC, which airs tomorrow at 8.30pm on BBC One, she said: “I just got really worried that I was never going to make anything that anyone liked again. It was just being self-critical of myself, it was nothing else other than that.
“And also I started to wonder, maybe 21 being so successful is enough for everyone?
"But I realised it wasn’t enough for me. So, sorry, I’ll make your ears bleed again.”
The 27-year-old made her comeback in October with chart-topping single ‘Hello’, while her third album, 25, will be released tomorrow.
After Norton asked her whether she was more “guarded” and “self-protective” with the new album, she said: “Yeah I was, just because last time no one was waiting to try and work out what my songs meant.
“I was so open about what the songs were about before, and I hurt people’s feelings with that, a little bit.
"Being so honest, and being a bit more immature then than I am now, I was like, ’I don’t care, like, he broke my heart, whatever’, and stuff like that. So I felt quite bad about that.”
Adele, who has a son Angelo, 3, with her partner Simon Konecki, added: “I have a lot of different things going on now.
"I have a family that I love, and I’m a lioness, and I want to protect them, and stuff like that.
"So I’d also like people to interpret these songs how they want to, rather than me shoving it down their throats.”