Especially for niceguy - and anybody else working on change (I think that's all of us!) - and me (this is partly my own personal therapy!)
You may have seen other posts in which I advocate using baby steps. I have found it too easy to beat up on myself and get negative when I attempt to make a big change (or set of changes) in one giant step. A mentor reminded me recently of the old riddle - Q. How do you eat an elephant? A. One bite at a time!
If I commit to making a change, and especially if it looks like a big change, I find it useful to choose a first step/goal that I feel sure I can accomplish. Then as I take that step I have an experience with success and can move on to another step with more confidence ('faith,' if you will).
A goal to ALWAYS think positively is a BIG goal - unattainable without a careful definition of 'thinking.' Negative things and thoughts will always be a part of your life. Otherwise, how could you identify what is positive? Opposition is a necessary part of this life! However, if you define 'thinking positively' as the action you take in the face of challenging circumstances, then I think it is quite possible to successfully create a positive approach to life.
I have found that noticing feelings is a wonderful early step. Ask 'How am I feeling?' I may be feeling tense, or discouraged, or over-whelmed - or I may be happy, peaceful, enthused. Becoming aware of your feelings is a step towards doing something about them. Congratulate yourself each time you find yourself noticing how you are feeling! I find journaling helps me remember that I am doing it.
Noticing thoughts is another early step. Ask "What am I thinking?" Feelings are the results of focusing on some thought.
Becoming aware of the thoughts that are in your mind and heart is another step towards doing something about them. Again, congratulate yourself each time you notice what thoughts are producing your feelings - and that means positive thoughts and feelings as well as negative ones! Again, journaling is a good reinforcement for me.
Once I notice a feeling and find the thought that is triggering it, then I can choose what to do with it. Since I want to enhance a positive feeling, I can nurture it by pursuing that line of thinking and enjoying the experience. And since I want to eliminate a negative feeling, I can get rid of it by ignoring the negative line of thinking and taking it in another direction.

Nobody knows better than you do what you can trust yourself to do. Niceguy, you ask, "do you think that my intention or law of attraction could do something wrong because i am taking the decision only for 25 days not for whole life?" It is a good goal to work on trusting for 25 days since you feel you can commit to that amount of time. But you must think of trust as the reasonable way to live. Keep a written record of your positive experiences with trusting. Do not write down anything about mistrust or fear. Focus on the feelings and ideas that come when you trust. When you review your record at the end of the month, you will see what positive things trust has brought into your life - and make your decision about continuing with it.
Wishing you may create peace and happiness,
Margaret Ida