Is "Fake it 'til you make it" an acceptable Buddhist behavior?

I would like to mention the difference between realities. We have conventional reality and ultimate reality.

Conventional reality is based on concepts such as "I, Me, Self, Persons, things, entities, animals etc." Here there is a Self, an experiencing entity, a creator of kamma.

In ultimate reality there is no concepts, meaning that there is no I or Self. In other words there is noone to blame or to restrain.

Regarding the past. What have been done have been done. Pondering about it is not conduceive to ones practice. Reacting to these mental formations only serve to bring one away from the Present moment and into delusion.

Remorse, regret of the past, thinking about what could have been done differently belongs to the 4th hindrance, ie. Restlessness and Worry. The hindrances are what keeps on from developing in meditation. You can read more about them here.

Lastly, in buddhism we try to develop a non-stick mind. A mind that does not cling or have aversion. We practice non-reaction, non-interfering with phenomena. Instead we observe them and learn from them. We observe phenomena and let them show us their true nature. When reacting to them, e.g. reacting to emotions we provide them further fuel to burn. We strengthen them. Its like taking a magnifying glass and concentrating the sun rays. They get stronger. In the same way if we have aversion towards them they also become stronger and grow bigger.

When just observing phenomena without interfering they are not being provided any fuel. Instead we turn them into objects of observation. We make them into the soil that will nurture our spiritual growth. So when these mental formations arise you can observe them and note them (Mahasi Sayadaw Tradition) in order to realize their impermanence, unsatisfactoriness and uncontrollable nature, i.e. the 3 signs of existence.

I'd say NO. Buddhism is, if anything, about practice, practice, and more practice. If you don't do the work, results will never appear. Your expression of emotion was entirely appropriate, even expected. A meditation retreat is a challenging thing, and if it had had no effect on you, I'd have been more surprised.

"Fake it till you make it" is one of many views a disciple could endeavor on his way to liberation. It could be a good way to gain some knowledge/wisdom. Once knowledge/wisdom is gained, sooner or later one abandons the view "fake it till you make it".

You have done right by not restraining your last public display of emotion.

Even if that emotion was coming from your ego, it should not be avoided. The reason the emotion came is to learn why it came. The whole universe is here for you to see the path leading to liberation. Even if you probably don't see it this way, your emotion was one of the many experiences that lead you closer to liberation.

The idiom "Fake it until you make it" or the recent turn of phrase "Fake it until you become it" used by Amy Cuddy in a Ted Talk about body posture and how it effects your psychology seems to focus mostly on the concept of confidence: http://www.ted.com/talks/amy_cuddy_your_body_language_shapes_who_you_are

In that sense in the west there seems to be an interesting idea that confidence is key to success and therefore one should be confident at all times. This is of course not possible and there is where this idiom comes in. The idea is that if one act and behaves as if one is confident, one becomes more confident by way of training. in other words: "embody as best as you can, that which you want to be."

If taken in this way the idea may be applicable to Buddhists, not waiting for confidence, not being paralysed by uncertainty but practising being the person you want to be. Note that this is not the same as ignoring ones feeling or suppressing it. Nor is it a mode you don't allow yourself to fail.

I agree with the other answers that you did not act inappropriately. As long as one does not flinch because of it, or see it as a failure.