Always and regularly SAVE PROJECT AS, giving it a unique name or number. That way, the save won't overwrite earlier versions, and you can revert if necessary.
Also, if you make a mistake, press <ctrl-Z> to backtrack - step-by-step. (Same as EDIT|UNDO) Then try again.
If multiple adjacent clips are selected (highlighted) they will drag together.
To drag an individual clip to the left into a gap, first click in an open area to the right of the sequence, to be certain that only the clip you want to drag will be selected. Then click on that clip, hold down <shift> and drag left. The other clips to the right should not be affected and a new gap should appear after the clip you dragged.
To drag clips to the right and not have them snap back, first select ALL clips that are to the right of the one you are dragging, click that left-most clip, hold down <shift> and drag right. All the following clips should move right and leave a gap.
Using DELETE (ripple) will automatically snap clips to the left, with no gap.