I always imagine having a good life. Never happened. So I don't think manifestation is real.
I feel it’s not unless you start acting and working in that negative way, confirm those thoughts.
It’s important to challenge your negative thoughts and have a positive perspective that empowers you to do things in a productive way.
Look for a therapist. There are various techniques they can teach you regarding intrusive thoughts. There is also a youtube channel 'Therapy in a nutshell' which have some videos teaching techniques how to deal with intrusive thoughts.