But after some months, the challenge is kind of gone and I just do the same thing again
Be careful, that's an FO-for-life attitute right there. not that you seem to care much anyway.
If you think that the challenge is gone after "some months" then you are obviously not doing what you should be doing. ie LEARNING
Your company has employed you to assist the captain in the safe operation of the flight however your other objective should always be to absorb as much experience and knowledge that you can from every flight.
just think of this period in your career as an apprenticeship.
There is a lot more to flying than you are currently aware.
Many FO's who struggle with their upgrade have wasted their years sitting in the Right seat. They think that the time to learn is when their turn comes for upgrade. By then it is too late.
if you want to continue your career as an airline pilot set your self the goal of learning something new from every flight. try to always stay in the loop. ask yourself 'why did the captain make that particular decision' and what would you have done different.
Flying is far from boring for those who are doing it right.