Last night I was in Cockermouth watching a collection of films about climbing. The climbers were explaining that one of the compelling things about the sport was the existence of doubt. The doubt about whether or not it was possible to get to the top of the route that you decide to climb. As you haven't climbed the route before you do not actually know exactly what lies ahead. You don't know how steep it is, how good the holds will be, how slippery or grippy it will be, how much effort it will need. So although you might set out believing that you can get to the top there is always doubt. And with that comes the fear. However by overcoming the doubt and fear you can climb to the top and feel the exhilaration of reaching that pinnacle of achievement.
Most people set goals and we don't actually know what does lie ahead for us, just like the climbers. However much you believe in your own success, you cannot control the world and those around you. So there will always be room for doubt. What will get you to the top is the willingness to just give it a go and see what happens. The climbers fell off their routes lots of times last night but they didn't give up, they just got back on and tried again.
So when it looks like it might be going pear shaped, stay tied on to your rope and give it another go.