When I say act as if… do I mean act the opposite of what I feel?

No, I am talking about your mind as a child; sweet and naïve. Remember when you were a child and you had purity of thought. You said whatever came into your  head and some people laughed and some were embarrassed that you said what you did.

Children may not always be accurate with their interpretation of what adults are saying or doing, but they are truthful.

Are rocks and trees opposites? They both stand still and strong but trees are full of life; they sway with the wind.

Your Purity
When you learn to be truthful about yourself and everything that you do, you do not have to pretend; words come out that are truthful and honest.
When you begin to realize that whether you are sad or happy, it is the same. When you do not act as if they are different entities; things change for the better.

On Speaking the Truth
In her book Yoga of the Heart by Alice Christensen, she speaks on practicing purity of mind, body and soul.

And when you begin a lifestyle change and you are practicing keeping your body nourished; you also need to clean up your soul and mind too.

Mind Clutter; trapping you into ruminating on the past or future

When the mind is full of clutter, it cannot balance itself. You have to clear out those bad thoughts from the past, and stop yourself from the worry of the future because everything in life unfolds as it should.
When the mind has no clutter in it; it can speak honestly, and sadness or happiness are no longer opposites; they are the same because it does not matter what comes next in life; what matters is the present moment where you are a helper to others and yourself.
Sadness and happiness seem like they are opposites but they are really the same.
And truthfulness is what you always speak in acting as if… Because this kind of acting is not an act, it is truth.
You do not have to remember what you said last, because you will never be caught in a lie when you act the same and tell the truth; no matter what happens in your life.

Purity Thoughts taken in part from: Christensen. A. (1998) Yoga of the Heart. Rondale Press. NY. NY.