If you want to change, you must first resolutely get rid of the defects of your vital being, persevering steadily, however difficult it may be or however long it may take, calling in always the divine help and compelling yourself always to be entirely sincere.
As for fitness and unfitness, nobody is entirely fit for this yoga; one has to become fit by aspiration, by abhyāsa, by sincerity and surrender. If you have always desired the spiritual life, it is the psychic part of you that desired it, but your vital has always come in the way. Establish a sincere will in the vital; do not allow personal desires and demands and selfishness and falsehood to mix in your sadhana; then alone the vital in you will become fit for the sadhana.
If you want your endeavour to succeed, it must become always purer and more steady and persistent. If you practise sincerely, you will get the help needed by you.
Ref : Letters on Yoga Vol. III