Today I've "rejected" an edit to a question, but came back to it later only to realize that the reason I did that wasn't right. I would like to withdraw my vote, but I cannot do it. The same, I presume, happens with other similar activities. (It took me more than a year to find out how to withdraw my vote on a question without downvoting it, so maybe there is a way, just not the one easy to discover, in which case, please treat this question as a request to make this functionality more obvious).
If the edit was applied even though you voted against it, there is nothing to revert, is there?
If the edit was rejected eventually, that can mean that some other reviewers voted to reject it. If you really believe that the edit was necessary, you can always apply it yourself, but you will then be bypassing the other reviewers.
You can view the status of your reviews (including the eventual action that was taken, and what other reviewers voted) by clicking the "recent reviews" link of the relevant review queue, then "history".
The same applies to other review activities: see if your vote had consequences. If you think these consequences should be reverted, engage the process to revert.
By the way, for voting queues (for example a close vote), your vote may have more influence than only 20% of the decision, because some people will think "eh, 4 guys already voted to close, who am I to disagree?". So you can be a bit less shy in starting a reopen vote if you feel that it is needed: in any case, the action will require a voting majority again.