I believe the BIGGEST challenge to achieving success in our lives is ourselves. By that I mean, the biggest obstacle is the fear of trying and the self doubt that we hold in our hearts. Because once we overcome those inner demons, nothing can stop us.
If you 'know you can do it', the opinions of those who say you can't will not be able to stop you. Obstacles such as training or education, etc. will not be able to stop you once you decide you can do it. Because then you will realize that you can go out and get whatever training and education you need, etc.
This then applies to ANY OTHER obstacle you meet. Once you realize that you are the only person who can stop you, then nothing else can stop you.