For years I trained in classical flute. Anyone who has ever played a musical instrument knows how frustrating it can be when you hear how something should be played in your head -- the notes, the rhythm, the volume, articulation, but you just can't play it as it appears on the page. You keep stumbling over it, missing the same notes or timing again and again. One day at my flute lesson, I stumbled into one of these road blocks in the middle of a complicated run, and it was clear that I was getting frustrated with myself. My teacher stopped me as I was about to try it again and told me that this time I should make sure I mess it up.
Mess it up? Wasn't that what I had been doing -- wasn't that the problem? She told me to intentionally mess it up, let myself mess it up. She knew that by allowing myself to butcher the music, I wouldn't put the unnecessary pressure on myself to play it right the next time. Oddly enough, that bar was very easy after messing it up on purpose a couple times!
Voice-overs are just the same! Especially when you are starting out as a new voice-over talent, it can be difficult to produce your "natural" sound when you're busy thinking about what words to emphasize, not duplicating the rhythm in each sentence, billboarding, and enunciating the words clearly. Voice-over work, just like playing flute, takes hours of practice practice practice. The good news is that you can have faith that you will get better with practice. One of the more rewarding benefits of recording practice reads is that you will be able to hear yourself make progress with each read!
When you do hit a stumbling block -- whether it's a tongue-twister of a script, or you just don't sound like you think you should, try messing up on purpose. It can actually be fun to take a voice-over job and read it as if you're doing a caricature of the part you are reading for. You may even discover that your phony version actually nails the sound you were going for to begin with. Over-acting a part can sometimes reveal that the reason you weren't getting it right to begin with is because you were under doing it.
Messing up all the time can certainly damage your ego, but being comfortable with messing up will alleviate the pressure to get it right the first time. Sometimes it takes messing it up royally to finally get it right!