For you to plan to propose to a woman, it means you love her so much and you are ready to take her as part of you forever. The courage you need is in the love you have for her. There is no guarantee that you will not be scared but you know that if you do not propose to her now, you are losing out on more precious time with her which can never be regain.
If you are in good terms with the girl, it's most likely that she will say "yes".
Speak to one or to two friends to encourage you more on why you should go on. This will encourage and inspire to go ahead with your plan.
First, you have to understand that courage is no guarantee that you won’t have any fear or trepidation. Sometimes courage is going ahead even when you are scared. You have to envision a positive outcome. You might be afraid of your girlfriend saying no, but if your relationship is stable and you are both on the same page, chances are you will hear a yes. Sometimes even a little inspiration can help.
Maybe watch your girlfriend's favorite romantic movie, preferably one with a scene of a man proposing to a woman; when you go to pop the question, pretend that you and your girlfriend are the couple, and you are re-enacting the scene from the movie.