Letting someone hit you in the face is kinda a half-way-there solution.
Here are some tips I saw in another forum:http://www.mmashareforums.com/striking-f19/how-do-i-avoid-being-tentative--t30059.html
Offense is the best defense. And if you spar, you'll get hit no matter if you like it or not and during the process you'll learn to accept it as a part of the sport. No matter how good you are, you will ALWAYS get hit from time to time. It's just the way it is. The faster you accept it, the faster you'll stop being tentative.