I would suggest focusing on your limiting beliefs / negative thoughts first. Really try to go deep and nip those bad boys in the bud. Work on your mental diet. Get clear about what you want. Be patient and kind with yourself. Try to practice gratitude even if it feels forced at first - it will start coming more naturally eventually.
It is hard at first for sure and when I started doing it I would literally repeat things like "it's all good" in my head thousands of times a day. After about 6 weeks of this I didn't need to do it anymore, am way more focused and notice immediately when a negative thought pops into my head and just let it go.
I highly suggest meditating if you don't already. I am an extreme over thinker and cannot focus, like ADHD level, but consistent meditation has really helped me.
There are so many youtube videos out there, but I suggest picking a few "coaches" that you like and sticking with their videos otherwise it can get confusing.
Good luck!