Is it possible to try 'too hard' in public speaking?  Most definitely.  Whereas that particular group of people is considerably smaller than the underachieving group (those who are not well prepared), the single greatest problem for the overachievers is in their overachievement:  it prevents them from being themselves in public speaking.

Those who try 'too hard' put limitations and restrictions on themselves which, in the long run, defeat their purpose because they are trying to achieve perfection.  In a live performance, perfection is not possible.  Excellence is however.  Excellence allows for mistakes; perfection does not.

And, the only way to achieve excellence is to know your material.   That does not mean the memorization of your speech or your presentation though.  The problems with memorization are two-fold:  the possibility of forgetting your material; and, the likelihood of sounding rote.  Whether you are giving a presentation or delivering a speech, your goal is to sound like you are talking to your audience.  Memorization does not allow for that because memorization does not allow for you to be you.  When you memorize you are only 'spitting out words', much like those inconsiderate people who call us during dinner to try to convince us to change our phone plans.  They never communicate with us.  They talk at us, not to us.

Another example can be found among those playing a musical instrument.  The musicians who memorize their music stand a much greater chance of forgetting than those who 'know' the music.  When you know the music, the music is then in your fingers, or in your hands, or in your voice.  It becomes part of you. 

When my college choir was on the road singing in various cities or countries, we never practiced the entire selection of music before the concert.  Having worked on this material for months in advance, we knew the music.  Our choir director would do a few bars from each selection to warm us up and that would be all.

You don't need any added stress when you are public speaking because the act itself creates more pressure than most people want.  This is why it is man's greatest fear.  For overachievers, the additional pressure they place upon themselves has the opposite effect of what they desire.   It creates added stress, added tension.  Unfortunately, this additional tension can affect other areas of their lives as well because the pressure of the presentation is always on their minds.

My advice if you are giving a speech or presentation is to begin practicing the material a few weeks before the event.  (Yes, I said a few weeks!)  Don't wait until the day before your program to begin this task.  Go over it a few times one day and then set it aside for a few days.  Go back to it a few days later, practice it several times and then set it aside for a few more days.  In the shower, practice your opening so that you feel very comfortable with your words.  [While not an advocate of memorization, I do recommend memorizing the first 3-4 lines of your material or the first paragraph to ensure that you can begin your speech as successfully as possible.]

So for all you overachievers, stop obsessing.  Stop seeking the unobtainable.  If you prepare in the manner described above, you will know your material when it is time to stand at that lectern and deliver that speech.  Your goal should be to do the best job that you can do.  Forget perfection; go for excellence.