I checked-in some changes by mistake. Now the changes are in the outgoing changes, but I don't want them to deliver. How can I return them back to the unresolved? If I discard them they vanished from my source code and I lost my work.
You can still discard the change set: see this thread.
By discarding the change set, the changes in it will be reversed. This happens to be the addition of the files once the initial share was performed.
(hence the removal of those files)
To avoid this, you can disconnect the project then discard the change set.
(when you disconnect the project, select the option which don't delete the local files)
This kind of behaviour is intended as you would want to discard a change and have your local shared content to reflect the same state as your repository workspace.