I've been studying Zen Buddhism for a little over four years. The first year was exploratory -- starting out sitting once weekly with an informal group and ending up sitting daily on my own with weekly group sitting. The second year I began practicing every morning with a sangha at a non-residential Zen center. The third year however I was unable to attend every morning practice due to personal schedule issues; but I was still sitting daily on my own and would attend at least one practice period with my sangha each week. The fourth year I had to move to a location where there is no formal sangha and I am now practicing completely on my own.
My experience accords with George's statement that in general, one must dedicate oneself to a good teacher, or at least a sangha, in order to find truly sincere practice.
One thing I learned that second year was that the structure which is imposed by formal training really helped me learn how constraints can be guides, and that the discipline is necessary in order to truly move freely in life.
Now that I do not have a sangha within any reasonable proximity, I can see why they call it the third jewel. Peer pressure can be positive sometimes, and practicing alone is extremely difficult.
I will never give up my training but sometimes I feel like my practice needs practice.