Tai Chi is mostly mental training. OK, I would be lying if I said that the physical is not important.
There is a fair share of hard physical training to be done. You can't get away from it.
However, the mind plays a key role. The mind directs the movements, it directs the intention and much more. As an example, each posture has a beginning and end. The mind must link the postures together smoothly without fudging the beginning and end. The mind provides the continuity. When issuing energy the posture may stop, but the mind must keep the intention going. The saying is that the lotus root may be broken, but the mind intent is not.
The external ( form ) and internal (mind intent ) must co-ordinate.