By thinking too much, do you mean you are being too skeptical or do you mean that you dream real big and then do not take the needed action to reach to that dream. Or you feel that this too impractical thing to happen?? Kindly clarify this will be able to answer your query better

This overthinking problem of yours can be a symptoms of pure obsessions, which are found among obsessive compulsive disorder clients. However without jumping into a conclusion i would suggest you to visit a clinical psychologist and get your detail psychological evaluation done. And also take some sessions which will help you to gain insight to ur problems and fond out way to cope. Clinical Psychologist do not generally suggest clients medications, untill its very necessary. Their approach is mainly psychotherapeutic.