How you do in technical interviews often has little to do with how good of a programmer you are. Modern technical interviews (in Silicon Valley, at least) tend to focus on a certain category of computer science and mathematical questions, rather than skills that would actually benefit an employer.
On the bright side, something of an industry has been built up around preparing people for tech company interviews (similar to the SAT prep industry). Check out Gayle McDowell's Cracking the Coding Interview[0], CodeKata[1], or Interview Cake[2].
[0] https://www.amazon.com/Cracking-Coding-Interview-Programming...
[1] http://codekata.com/
[2] https://www.interviewcake.com/