"전 이렇게 될 거라고 생각하지 못했어요" = 저는 (그것이/제가/...) 이렇게 될 것이라고 생각하지 못했어요. => I had no idea it would turn out this way. (If "it" is the subject) => I had no idea I would end up this way. (If "I" is the subject) - (그것이) 이렇게 될 것이다: It(그것이) will(것이다) turn out(될) this way(이렇게). - 이렇게 될 것이라고: 이렇게 될 것이다 + 라고(quoting particle - "that...") => That it will turn out this way. - 이렇게 될 것이라고 생각했어요: I thought it would turn out this way. - 생각하지 못했어요: I had no idea that ... It didn't occur to me that ... - 이렇게 될 것이라고 생각하지 못했어요: I had no idea it would turn out this way.

전(I) 이렇게 될거라고(that it would be like this) 생각하지 못했어요(couldn't think/didn't expect) = I couldn't think of that it would be like this. =I didn't expect it would be like this.

I didn't think It would happen this way

What you say is that I did not think that this would happen. It just happened without your expectation.