You've heard this before, right? You can' be objective about your own voice. You need experienced professionals to guide you to a good performance, to really get the best of what you have inside of you out and into the mic. I call bullshit.
I've heard this advice several times in the last few years. Usually by somebody who's either a vocal coach, a demo producer, an agent, or a combination of the three. I liken my voiceover skills to my guitar playing skills, and I improve on both of them in pretty much the same way. I record myself constantly (thanks to voices.com, I'm never wanting for new copy to practice on), I listen to it, I develop an understanding of its technical, emotional, and subjective flaws, and I work to fix them. This is how I've increased my sensitivity to mouth noise, how I've shifted my pitch to what it ought to be, and how I've discovered when I'm being truthful and when I'm full of shit, and how to work to avoid the latter.
This advice is like telling guitarists that it's impossible for them to get better without a teacher, and I know from teaching myself guitar that that is completely untrue, and is probably being told to you by someone who wants your money. Which is fine; I'm not knocking coaching in general, as it can help you improve much more quickly in some cases and should be at least considered in your overall VO strategy. But saying you can't be objective just because your voice comes from you is disingenuous. Of course you can be objective. My goal for a dramatic piece is to engage the listener on an emotional level in a very short period of time, in a way unlike anyone else and yet is reminiscent of moments in my life that have touched me. That's an objective, and the components to achieve that goal are my pitches, lilts, tone, timbre, body language, etc etc etc. I build that moment out of the tools I have at my disposal, and if a tool isn't sharp enough, I pull it out and sharpen it. If my L's or M's are too wet, I stretch my mouth out and do more obstruction exercises or tongue twisters. If I'm not present enough, I move into the mic, taking care to over-enunciate slightly to cut down on the additional mouth noise. If I don't sound convincing enough, then I need to strengthen my connection to the script by associating it with something in my personal life that resonates with me. Just because it's acting and it's hard doesn't mean I can't examine its parts and work on it without help.
You know a good guitar solo when you hear it. If you actually study guitar, you build on how to make that solo happen. Voiceover is really no different. So, please, be objective about your voice. It's the only way you'll make it better.