Why should I prepare for UPSC? When should I give up without regretting? - Nice question.
Well first of all you should now prepare for it just because you have wasted/dedicated (depending on your perception) almost 2.5 years of your life on this thing called UPSC. Trust me there is no turning back now. Take up the challenge and amend your mistake and move ahead.
For the second question, you can never leave UPSC without regret in your mind. UPSC is just like a first love that you want to forget but can never actually do so.
Bro, UPSC is the toughest exam in India. Go through the previous year's result and you will realize the candidates who are getting selected have put almost 3-4 years of their life into it. In my personal opinion, it takes almost 3 years of dedicated and planned preparation to crack the UPSC. Even the people who are getting selected in the first attempt have their optional from their academic background or have been preparing for 3-4 years prior to their first attempt.
Even I was in your position once upon a time and it took me 6th attempt to get the first call of the interview. It's just because of our own ignorance and haughtiness that I will crack it without coaching and on the basis of self-preparation. I learned it in a hard way that without the proper guidance of a dedicated mentor its way to difficult to crack UPSC. So it's my humble suggestion that you should plan properly for your next attempt and find a senior mentor who has already gone through the process to guide you.