Obsessive ruminations are quite common and occasionally can become severe enough with significant impact which then makes it a disorder called OCD. Most obsessive irrational thoughts are down to certain vulnerabilities in some people leadi to development of certain dysfunctions like these. You need to start challenging your thoughts and start looking for concrete evidence supporting them. You will soon realize that not much evidence to support them. Then you got to start appreciating those obsessions in the context of your vulnerabilities or OCD, without reacting or suppressing them nor fighting with them. You accept them in the right context and let them be and carry on with normal life consistently, so that all these obsessions start to lose significance and disappear eventually.
Next Steps
If you find these thoughts too overwhelming and unable to control, consult a psychiatrist for an assessment and appropriate treatment usually using psychological techniques like cognitive behavior therapy, lifestyle modifications and at times medicines