So many deep rooted thoughts and not sure which one to work on first!

I have a lot of thoughts which are very deep rooted and I cannot imagine what it would be like to not have those sentences playing out in my head multiple times per day. I’m tired of suffering and really want to start working on changing them.
Do I work on the thoughts causing me to suffer the most?
Also, do I repeat the new thought over and over every day?
Thank you