How to keep a habit?

I have heard that if you keep doing sth once a day for 3 weeks, it would be u habit. Yes, i did, but sometimes i was frustrated, Why? I found my incentive was going to be shrank, my passion was cooling down, no matter how did i fuel it. But I have to hurdle it, i believe i can make it.
If you want to get sometimes, you would first give out somthing. Just like that you should first seek to understand, then to be understood. But i guess it is not a piece of cake.
I always say that nothing is impossible, unless you give up. Yes i know, you just dont agree with it. it sound like a simple, not mature and short sighted guy would say. If you see more, hear more, look into the reality, you easily found that the obstacles are everywhere, they are overwhelming, enormous, make you contract, make you shirk. You have no choice but give it up.
But i want to believe (not X-File) the most valuable thing can only be made with the longest, most ineffective way. "Pain is inevitable, suffering is optional". You dont believe it, my friends dont believe it, even my closest family, doesnt believe it. Okay, but i have my independent will, i want to believe it. Despite i feel really tired.