How do you overcome self-sabotaging thoughts when it's impossible to cut off people contributing to these negativities?

Practice meditation, this will help you to still your mind. When not in mediation (most of our day), take breaks, stop and ask yourself what you are thinking. We play these negative tapes in our head as a habit, by recognizing this we can become more aware of when we are doing it. Eventually you learn to let the thoughts flow through with out carrying on with them. Be kind to yourself as this isn't an overnight thing it takes practice.