If you force your retainer in will it shift your teeth back?

Probably not: A retainer is not an active appliance.. It is designed to hold teeth in place, not move them. If the teeth have shifted just a bit, and the retainer is tight, but still fits down all the way, then wearing it 24/7 (unless eating) for a few weeks can make things right. If the retainer does not seat completely, you are doing more harm than good. See your orthodontist.

How sad: To waste all that time, effort, and money. Before it gets worse go back to your orthodontist. A specialist can tell how much relapse has occurred and what can be done to recover. Forcing a retainer onto shifted teeth cando more harm than good. See an orthodontic specialist for advice, and go soon. Delay will just allow additional deterioration.