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http://www.paulgraham.com/identity.html
I can suggest three approaches to not feeling guilt over using other people's code:
1) Get "writing difficult programs" out of your system by writing some really difficult programs. Don't let it contaminate your more practical work. Optimize separately.[1]
2) Notice inconsistencies related to the guilt - can you do more good in the world with the time and money saved by reusing existing code? Know the concrete thing you're trying to achieve, and remind yourself of it when you feel guilt over trivial things. Another inconsistency: you probably don't feel guilt over using Intel processors or Linux or all the amenities of civilization.
3) The nuclear option - hundreds of hours of meditation. If you can notice the guilt over and over again, or repeatedly ask yourself who is feeling the guilt, you are almost certain to attain mastery over these unwanted thought patterns. See [2]
[1] http://lesswrong.com/lw/6z/purchase_fuzzies_and_utilons_sepa...
[2] http://sharanam.tumblr.com/post/865951869/kenneth-folk-a-sys...