KSP: How to move parts closer/farther in the editor

The VAB/SPH editor interface frustrates me to no end. I just can't figure out the most fundamental control of moving a part closer/farther from the camera.

Yesterday I nearly lost a half-built large ship as I misplaced OCTO, the tiny control module which was the root part. It just got stuck somewhere in the shelves in the background and I spent good half a hour looking for it before finally saving the (ghosted, uneditable) remainder of the ship as subassembly and starting a new craft. And how did I misplace it in the first place? Wiggling my mouse desperately trying to bring it closer to the ship in the center, as it floated somewhere in the corner between the gate and the shelves and would get stuck against either of them. An errant click, and it was gone for good. Before the Subassembly idea, I really considered taking a screenshot of the warehouse wall and posting here, "Help me find my OCTO", a "Where's Waldo" style puzzlefor Arquade users.

But tanks that just refuse to move close enough to snap into their designated places, parts that keep floating far behind the ship (and stay there as I zoom out), me trying – and failing – to game the system by sticking a part to the floor, turning the camera 90 degrees, moving the part to the other side of the ship, turning the camera again, lifting the part to its place… and finding it back behind the ship again, this is frustrating. I tried to figure out how the mouse movement affects whether a part is moving sideways, up/down or close,farther, but I couldn't recognize any regularity, parts switching between diagonal horizontal and straight vertical movement at seemingly the same gesture at random.

Could someone explain this comprehensively or give some good workaround tips? Maybe some mods that fix it?