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.