I'm starting to meditate again, mindfulness, and meditation learnt from a book. But my question is, why is the mantra given secretly and personalized to each individual by a master? Can't I just use any mantra or word? What's the difference and significance of it?
I think it's for the sake of authenticity to one's true self. But if you're asking those questions, then the question you should be asking is this: "Who's to say it won't work in the way I want it to?" Almost every brilliant idea is a remix of another. In the time spent debating the merits of a specified/personalized mantra, you could've tried a bunch of others that already exist.
If you're talking about mantra meditation, which is different than mindfulness meditation, it's because the sound of the mantra has certain "vibrations", and those vibrations are key to the meditative effect. The guru gives you a mantra tuned to your specific "frequency". The reason for keeping it "secret" is like why you're not supposed to hand out prescription drugs to people. Like the drug, it's meant for you and someone could be harmed receiving it from you, since you're likely not qualified to know its effects. That's the theory, anyways. Mindfulness meditation is completely different because it is non-conceptual. Instead of focusing on the mantra as the object of meditation itself, the object of meditation is a fixed reference point. When you notice yourself forming concepts instead of being just a detached observer, the object of meditation (like your breath), allows you pull yourself out of the trap of conceptual thinking, restoring a state of pure mindfulness and awareness. While it's true that methods of meditation are derivative, you shouldn't mix and match if you don’t know what you're doing, and you should find a qualified meditation teacher in whatever style you find appealing. Just like you shouldn't mix and match drugs unless you're an expert in pharmacology.
Your interpretation makes a lot of sense actually, thanks Regarding the differences in meditation, I am aware of it. The book and practice that I will be doing is a mix of midfulness meditation and this deeper mantra meditation (i believe it's from nishkam karma yoga) with a generic mantra provided in the book. So it's 15 mins practice twice a day, starting with a few minutes of mindfulness and then switching. If you have any tip or suggestion, please go ahead
I'm totally not qualified to make suggestions. Other than be careful and take some precautions. I've heard of people having psychotic breaks from mindfulness meditation, because pulling your brain into "non-conceptuality" can lead to disassociative states where you might start questioning whether reality really exists and losing the ability to function. That's why Buddhism focuses on devotional practices for lay people and reserves mindfulness meditation for people who are ready for it.
Instead of asking what is the right mediation for you, just try it. Sit down and be still, when you feel the stillness in you, you can observe your thoughts or there are no thoughts at the moment. Try it out over and over again and at some point you are in your own meditation, which cannot be explained by a book or s.th. else.