Instead of weighing each new distraction as it comes, make one decision in advance: everything is no until the current project is finished. Removing your own escape routes, such as going somewhere with no other options, makes focus automatic instead of a daily fight.
A habit that only happens when several conditions are all true at once will rarely happen, because all the conditions seldom line up. Rewriting the rule so that any single condition is enough to act turns a rare event into a frequent one.
Saying no to almost everything is a tool for when you are swamped with offers and everyone wants a piece of you. Early in life, when you have few options and can't know which will pay off, the better strategy is to say yes to everything, like buying every lottery ticket.
Saying yes to things you feel half-hearted about fills your time, so when a truly great opportunity arrives you are too busy to take it. The fix is to treat anything short of real excitement as a no, which leaves your time open enough to throw yourself fully into the rare thing that deserves it.
Asking what you want is unreliable; asking what leaves you depressed or feeling off course when you don't do it enough is a better guide to what matters. Your actual behaviour, not what you say, shows what you really value.