If you haven't committed the merge yet:
Save the files with your changes somewhere. Maybe zip up the working directory just in case.
Shelve everything with
bzr shelve --all
.Undo the merge with
bzr revert
.Redo the merge and commit it.
Unshelve. What was merged should be unchanged. The difference should be only your own, new changes.
If you already committed the merge:
Create a new branch from the point before you merged.
Redo the merge and commit it.
Copy the files from the other branch over the current branch. The difference should be only your own changes.