This can be anything from stagefright for a musician to a fear of public speaking. From bedroom nerves to just carrying out your normal job of work but fearing it won't be good enough. To ovrcome this we need to be 'in the zone', or 'in the flow', terms most people will be familiar with but may think that it's only for top achievers or people who have done what they do for so long that they're masters of it, not so. If you watch kids playing any sort of game, they're totally in the moment, they're in that zone. They may have only just started doing whatever they're doing so will be far from experts at it but outside of that game for them nothing exists. As adults we've accumulated all the self-doubt that goes with years of life 'experiences' so we get in our own way, that's the only thing that stops us getting back to that nirvana state.
However good we already are there is always room to improve. Getting in the flow is imprortant, see above, and motivation to practice & procrastination is another aspect. When we turn something from a hobby into a trade or profession than it can take on a different meaning for us & the pressure is on to do it well which can take some of the fun out of it. On the other hand if we're getting paid to do something we like or even love, a dream that most people never achieve, then it frees up time to pusue that one goal. If you're worried that the pressure is getting to you and so affecting your ability to carry out whatever it is then it's time to look at some of the reasons it may be happeing.
This is just about knowing your material, and knowing how to deliver it in an interesting way. This all well & good for a seasoned speaker who is able to gauge his audience every time but for a newbie it can be terrifying. What we have to remember is what's the worst that can happen? No-one is going to pull out a gun & shoot you if you get it wrong (at least not in the UK). The worst thing is you'll feel like you didn't give your best, and the lesson learnt will be very valuable, enabling you to do it a bit better next time. Very few good speakers are born that way, they usually have to go through a long process which takes them out of their comfort zone on a regular basis. You can short circuit that process to a certain extent with a few hypnosis sessions so your subconscious doesnt't sabotage you on the day with a self-doubting voice.