The same fire or single-minded thought is needed in a Mantra Sadhana, when you become desperate to get success in a Sadhana that's when you will be successful. Unless and until you don't have the fire in yourself in your heart to do the Mantra Sadhana how can you even get success? But how will you do a Sadhana? If you want to watch TV, play games, etc.
You want to eat unhealthy foods just for the sake of the taste and get ill and then few days later you again start eating fast foods and getting ill and keep on repeating this procedure for your whole life and then look for Mantras to cure such illnesses.
If you want to do a Mantra Sadhana by following the above kind of lifestyle then even if you have a Guru you will not get success even in a 1000 years.
Mantra Siddhi is not a joke that one day you just sit in front of your Isht Dev, light a Diya and incense sticks and do the Mantra Japa. Where is the dedication and devotion in this? and most importantly where is the faith in the Mantra? where is the determination that this Mantra is my sole purpose in life right now?
Not everyone but many would think "I have already chanted and got failure in so many Mantras so it is not really a big problem if I have got failure in this Mantra too".
I am not insulting anyone but believe my word that unless you don't have burning desire to gain Siddhi over a Mantra no matter how many times you try you will never get success in it. That is why Mantra Diksha is very necessary.
If I as a person, gains knowledge of a Mantra and shares it with a Shat-Buddhi person who claims to be doing various Sadhanas; that person's destruction is imminent. Every thing in Anushthan has Sanskar. There are 16 types of Mantra Sanskar by default like Keelan Utkeelan,and a lot more. Unless you don't do this, the Mantra will just not work.
Every Mantra shared by me requires these Mantra Sanskar. However, if a person is devoted, loves the Isht Dev or Devi and has a really strong faith in the Isht Dev and his Mantra, the Sadhana will be successful even without Mantra Sanskar because such a person's devotion is true. Again, all of it still lies in the hands of god and depending upon your devotion he may or may not give you Mantra Siddhi.
And I don't even understand on why one person wants to achieve Siddhi over all Mantras and always use the most powerful Mantra in each task? One single mantra || Om Hanumate Namaha || has the power to give me Pratyaksh Darshan of Hanuman, bring the sky down to earth, kill someone, cause enmity among people, get Moksha, get wealth, go to Brahmalok, get Divyastra and much more But most people will say I am a fool, how can such a simple Mantra give Prayaksh Darshan of a god like Hanuman. This argument is same like a Pandit arguing with a scientist. The scientist will say there is no such thing as Mantras and all that is foolish and fake while a Pandit will say the opposite. It isn't the scientist's fault. He himself doesn't know about Mantra and that is why he is disrespecting it. and this is when Guru comes in. One Mantra can do everything, it all depends on how you use it.