From my own experience I can say that the above is true. I tried it several times and noticed that I started to feel different by behaving differently.
I wonder if that is not one of the reasons that yoga and tai ji can work so restful (after all, the exercises are generally performed calmly).
Yes, this is indeed very true. Often enough I notice that I immediately become happier when I behave happier. Retaining is an art in itself, especially because many people think I tend to exhibit certain behavior unconsciously. But that will also become easier as you practice it more often.