The current issue of Running Times (September/October) has an article titled Never Satisfied written by Richard A. Lovett. It's a very well written article about runners trying to achieve perfectionism and how that's not exactly the best approach. This article is very applicable to triathletes... especially since triathletes tend to be type A people (of course not all triathletes are).
Also, striving for perfectionism sets yourself up for regular disappointment and sucks the fun out of training and racing. A lot of people focus so hard on just the results rather than the experience of the race or training. I fully agree with Lovett, "...shift from thinking only about the outcome to focusing on the drama and experience of the entire event: from training through race-day preparation, strategy, tactics, and everything else that goes into a race."
When in a perfectionist mindset you are more focused on the outcome. Shift your focus to mastery instead. By doing this it will allow you to still become a better athlete without the mental stress of having to have everything perfect.
Only having the end goal in mind... i.e. 2:50 for a 70.3... will most likely set you up for disappointment. Instead set two goals for a race. Have an A goal and a B goal. Your A goal should be something ambitious but realistic and the B goal should be something you know for sure you can achieve and be satisfied with.
Every great athlete and achiever has had many failures and some pretty big ones. You learn from them and get better because of it. Often times you learn more from your failures than from your successes.
So stay positive, make realistic goals, enjoy the experience, and smile!!