Mindfulness involves noticing, without self-criticism or judgment, the bodily sensations, thoughts, and emotions that are coming up. Check here some ways to avoid overthinking: https://mind-globe.com/ways-lessen-runaway-thoughts/
You might think being alone makes you lonely, but that*s not entirely true. Being surrounded by the wrong people is the loneliest place in the world. Before you diagnose yourself as an unhappy person, first be sure that you are not simply surrounded by negative people constantly trying to bring you down to their level.