Personal development is very ... personal. Each of us is unique personality with our own history and right now we are at our own personal position. It's like a computer game - SuperMario stands at the edge of an abyss and he has to jump. Thought you always press the same buttons the jumps would be different when Mario has to hit the stream of uplifting hot air and when he must catch a liana to swing down to the other end.
In your own life you have to recognize where you are and how to move forward. Someone would jump over the river, others will swim, hire the boat or go find the bridge. You might decide that you rather turn back and stay on your side of the stream.
With personal development its kind of similar. There are many techniques and tricks but not all of them are suitable for you in the current moment. I mean they are all good, but to achieve the best results you have to apply them your own personal way.
Great example (from financial development) are stock markets. Many people try to invest the same way like 'gurus'. They try some free trading without any risk and it works. Once they start to invest real money it's a catastrophe. Why it worked for the guru and not for us? The guru had different experiences, his feelings connected to money differed too and even he probably failed few times before he got enough confidence to teach his techniques.
There won't ever be a situation when you can use something and be absolute sure about the result. But don't worry, feel good, use your intuition and you'll find the right path for you.