Psychology Today excerpt:

​

Why would making our goals public reduce the likelihood of achieving them?
Making goals and any progress toward implementation generate positive rewarding feelings. In order for these emotions to be motivational until our goals are realized, the reward has to be time-released. Our brain cannot afford to be a logical mathematical reality machine at all times, that would be too costly. When our brain is tricked into thinking that the goal was achieved, it stops investing energy towards further implementation actions.
When we publicize our goal intentions, and others acknowledge the awesomeness of such “potential” changes, we get our dopamine reward all at once (In a previous article, I discuss how dopamine aids the most resistant type of motivation “want”). The more others admire our goals, the more dopamine rush we get, and the less likely we are to execute the future necessary actions to implement them.  Therefore we deplete our “feel good” gas, keeping us from reaching our final destination- our goal. Furthermore, publicizing our intention to succeed gives us a “premature sense of completeness” (1). It signals the brain to move on. In other words, If the brain believes that you have reached your goal, it might inhibit the specific brain circuits related to further pursuing this goal.
This is also true if we announce our success prematurely, this stagnates further progress towards the final larger success outcomes (2). That is why many of us might fail after bragging about reaching a sub-goal such as eating a couple of healthy meals. To the brain this means “goal accomplished!”. Even though, our initial goal of losing 30 pounds requires eating 1,000 more healthy meals, working-out, and viewing our favorite desserts as poisonous substances.
Secondly, we all have a basic need for competence, which is the basic desire for effectiveness, ability, or success. Much of our behavior is motivated by hope for competence and fear of incompetence (3). This need motivates us to sharpen our skills, change old habits, go to therapy or take new courses. And research shows that the more incompetent we feel, the more we desire to recite our competence goals in front of an audience. The more the audience compliments our identity goals, the less likely we actually work on our goals to become more competent. In other words, when we publicly set goals to become a more competent person in area X, our brain gets tricked into thinking that this future competent self is actually our real current self.

Read the whole article and the "solutions" to what  
-----https://www.psychologytoday.com/us/blog/neuroscience-in-everyday-life/201801/why-sharing-your-goals-makes-them-less-achievable