I used to sing from my own song book claiming, like some above, that I didn't have the capacity to memorise lyrics, but I found it got in the way of 'performance'. Doing two things well (singing and playing, contemporaneously) is difficult enough, adding a third - reading - is just too much, and the quality of my singing/playing was suffering. In addition to which, reading prevents eye-contact with the audience (something I regard as an essential element in the performer/audient-bonding process).

So I made Herculean efforts to learn my stuff, and I'm able to perform without the safety-net 99% of the time now.

Unfortunately, during a gig last Saturday, I hit the remaining 1% - had a brain-fart and completely dried after one verse and a chorus. After four or five seconds of silence, during which my entire life flashed before me, I admitted to the packed room that I couldn't remember any more of that particular song, and went into the one that I 'keep up my sleeve for such an occasion' - one which I've sung for many a year and could sing/play in my sleep. Result......loud and prolonged applause! :-) :-)

Learn your stuff. Leave the book (or that f**kin' i-Pad) at home. There's no shame in forgetting words. Everyone does it. Even the pros do it occasionally.