It's horrible to acknowledge any bad memories are true, and it does take time to piece things together. It sounds like a good idea to use grounding techniques or other strategies to help with flashbacks. Journalling or something creative can help a lot.
Flashbacks can't be "false" as such - they are a symptom of unprocessed trauma so something traumatic must have happened, and do often come in pieces (not exactly like the full cinema experience which goes from start to end). All I can say is that another person can't tell you exactly what happened, and it may be sometime before you really know. I'm sorry you are dealing with this.