Why don't I completely surrender to Krishna, even though I understand the real nature of the soul?
If I could answer that in one letter it would solve millions of people's problems ;-) Including my own. Here are some thoughts on the subject:
First, there is a difference between theoretical and practical knowledge. I may believe that I'm a spirit soul, that I'm different from this body, but when I stub my toe, I may become completely bodily conscious in less than a second.
Complete understanding that I'm not this body would be indicated by a dedication to satisfying the real me, the soul, by exclusively serving Krishna by various devotional activities, without any attraction or contemplation of material objects offering any pleasure. Since I can honestly say that now and again I consider deeply the possibility of finding pleasure in material objects, I can conclude that I haven't completely realized my spiritual nature, or my eternal loving relationship with Krishna. Perhaps you're also in that position ;-)
Even if we're not fully realized, we can at least be aware of our shortcomings, and develop a desire to become realized. It's up to us to accept the prescribed medicine, diet, and exercise regimen in order to gain the strength and wellness required to achieve our stated goals. Srila Prabhupada and the disciplic succession are wise physicians. They have prescribed worshiping the Lord, reading sacred books, associating with devotees, visiting holy places (including the temple) and chanting the holy name as the major curative elements in moving from theoretical to practical knowledge. Based on our faith in the doctors and the process they recommend—as well as our determination to follow their order—we can advance.
Now this idea of "completely" is tricky because our complete surrender is measured not only minute by minute but also over our lifetime. We may advance, and then we may slip, but the continued attempt to return to the path does not go unnoticed! It is a part of the required exercise for making advancement.
Patience, conviction of success, confidence that guru and Krishna are our well wishers and are arranging things so that we will come out okay, and also being humble enough to admit that I am small and the path is long; all of these factors allow us to have the proper attitude so that we can stay with the gradual development of Krishna Consciousness until we achieve success.
And that, too, comes in stages. We gradually acquire the pieces that, when assembled, indicate that we're absorbed in Krishna and willing to follow His order, and surrender. It's rare that this event is an immediate, one-time epiphany, although it can be. Such a sudden and complete transformation is very, very rare.