Well from my perspective over thinking something and having delusions are two different things, overthinking comes from anxiety and worries whereas delusions are fixed thoughts and ideas or beliefs that aren't likely true or completely untrue. For me I tend to stop overthinking when I accept that I don't have control over a lot of things and use a healthy distraction such as cleaning taking a shower talking to a friend or watching a show or movie and that helps reduce my anxiety so I'm not overthinking too much. With delusions, I can't really help but think them its a part of my disorder but I have learned that before I get really bad and realize that I'm manic I can ask someone around me if something is true or ask myself if what I think is true during my mania was something I believed to be true before mania. Hope that helps