I work with lots of nervous riders, having had two major injuries and being determined to beat the associated fears, i work through each riders personal problems with them and find the right route out the other side.
Being nervous is a common issue, but your nerves needn't be the end of the line for you.
The first and main point to address is WHERE the issue stems from. Can you pin point the EXACT issue?
For example: "I am scared to jump", this is so common, but is it the “jump” that is really the issue? Usually it is not the jump, it is an association with jumping itself. Perhaps you had a fall, you have immediately associated jumping with falling off, falling off resulted in a fright, and possibly pain, so your immediate response is :“i am scared of jumping”, when actually the correct identity is “I am afraid of the pain and being frightened like that again”.
So when trying to analyse your reasons for being nervous, look deeper than the surface, it is there you are more likely to find the root, and from the root you can rebuild your confidence.
So, staying with “I am afraid of jumping”, don’t push yourself too far too quickly, begin with poles, enlist the help of a trainer who understands your fears, begin again on a horse who is schooled and easy to jump, and finish the session when you are enjoying it and WANT to keep jumping, then you will want to do it again, if you go too far and get a fright, you will reaffirm the negative thoughts and convince yourself that your initial fears were warranted, reinforcing the nervousness.
The same principles apply to schooling, hacking and handling. Little and often is the key.
Riding should be a pleasure not a chore :)