God's plan for you doesn't change. Even if you happen to muck it up somehow, He would still want you to do it. And He'd even help you fix it. He probably won't reveal the plan to you all at once, too.
I know my calling, but the steps between here and there are pretty numerous...
If you could accomplish this calling by yourself, go ahead and doubt - it probably isn't a calling that God wants you to do.
However, if there is no possible way you could do it of yourself, then pray for God to take that doubt away and show you His plan for you.
Though, I may be making the word 'calling' and the phrase 'God's plan for you' the same thing...
I hope this helps you, even if just a little