For those who don't believe it:
Though it is good for me that now I don't have to fulfill my commitment, but I'd like to do it so can you bring it back?
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You most likely never fulfilled your commitment. As Energy Numbers noted, it likely isn't showing on your profile page since Lifehacks is past the 6 month mark of being in beta, and you never fulfilled the Lifehacks commitment so it doesn't appear under fulfilled commitments. There is no spot for "unfulfilled" commitments that are not longer current commitments, thus you don't see it on profile page.
From the Area 51 FAQ:
Once you've fulfilled your commitment on a new site by asking and answering a few questions, you'll be able to commit to another site. If you are very active, you may be able to re-use your commitment as soon as the second day of public beta. If you aren't active on the site, you won't be able to re-use the commitment until the end of the public beta, or until six months after the beta began.
How many questions/answers do you need to do to fulfill the commitment? I have no idea. I know on woodworking.se I have answered 2 questions and asked 2 questions. But I still haven't fulfilled the commitment.
I'm going to assume that only posts with positive scores are counted. So if you post 5 questions and 4 answers but only 1 of your posts has positive score you probably won't fulfill the commitment.
I'm not sure there is anywhere where the exact requirements are outlined in detail. So all you can really do is participate a lot, both asking and answering, writing high quality content and hope you get commitment fulfilled. I think it's probably by design that they don't share exactly how to fulfill commitment.