For more than 3 months, I have the maximum amount of commitments, but I'm not interested in being active on these sites anymore. How can I "step back" so I can commit to other, more interesting commitment-phase sites?
If the proposal is still gathering support during the Commitment phase, you can simply click on 'uncommit' button at the top-left of the proposal and your name will be removed.
But once the site is launched… no, you cannot revoke your commitment. That would betray the very idea of a "commitment".