meditation not done correctly and without guidance would mostly result in no progress.
You should try meeting Om Swami [0] to check for yourself the man who has attained spiritual adeptness with meditation – he'd be in Canada next month.
It's not mandatory to have a guru (teacher isn't a right translation of Guru) to follow path of self-realization however, having one certainly help. When someone asked Ramakrishna (the Guru of famous Vivekananda) similar question he said indicating a boat being dragged by a steamer "The boat can very well cross the river on its own but when it's attached to the back of a steamer, it crosses the same quite quickly." So the role of Guru is like that of a steamer in the sadhak (practitioner) life.
A practitioner really doesn't know his mind because the one who knows his mind can really subdue it. After all the modifications of mind is the only thing that has to be put under control so that it speaks only when you want it to. That's the state of self-realization.
0: omswami.com