Service to mankind is service to God.
Instead of that if you have negativity in mind, even simple works can also look like giants.
Self confidence is always required. Lord Hanuman is not aware of the inherent energy he is having. He started thinking that he may not be able to cross the ocean. Then all his fellow members told him that he can do it. Then Hanuman realized his energy, he told himself, I CAN DO It. He crossed the ocean, entered Lanka and seen Sita.
If you have a strong desire to do a work, you will be able to do that. What ever hurdles come you will over come and make the work successful. So positive thinking is good for all.
So start the day with a positive thought and say I Can, you will accomplish your goal.
In that regard self reminders like 'I can' and "I will' will help to reinforce self confidence. But if they are in excess and overdose or expressed outwardly often, then the result can be contra also. Even a small failure of such person will get inflated by adversaries and critics.