I Have Been A Team Leader For Three Years. I Had An Intern Under Me For Past One Year. I Trained Him And He Performed Really Well. Now, My Boss Promoted The Intern To A Position That Is Above

While the situation may be disheartening, I would suggest you not to focus too much on the promotion of your colleague. Instead, have an honest and positive conversation with your manager about your growth path. Ask for feedback on your performance, what's next for you, how can you reach there, any training or skill enhancement you should consider undertaking etc. Along with your manager, set clear goals for yourself, and then pursue diligently. I wish you success.

Firstly, try not to suffer heartburn here. Secondly, take credit for having trained him well. Somewhere his promotion directly reflects and validates your leadership skills of training and mentoring. Keep in mind that promotions are about the future, not the past. They are about putting the best person into an open position starting ‘today’, considering him to be the most appropriate fit for it and not going by his longevity in the company.
Alongside, You could also have a meeting with your boss to find out what you should improve to maximise your chances for the next promotion. And next time there is an opening coming up get proactive about asserting yourself as the front-runner. It’s an opportunity to propel yourself into action.