Never regret the past. Never.
I have promised myself not to feel that, because sooner or later I will see the "why".
Even though I've painted a picture using the wrong colours, I learnt that it was wrong. I'll not repeat it again. It was a lesson. Yes, there are no mistakes in life, only lessons. Everything happens with reasons.
And yes, the bad days will come again and again. But as long as I let the past go, I believe that I will have a bright future ahead of me.
Rasulullah (peace be upon him) said: "Seek out that which benefits you, seek help only from Allah and never say you can't do it. If any adversity comes to you do not say: 'If I had only acted in such-and-such a way, it would have been such-and-such'; but instead say 'Allah has decreed (it) and what He willed, He has done, for verily (the word) 'if' opens the way for the work of syaitan" (Muslim)
Let the past be the past, so that you will benefit the reason why God created tomorrow for you.
"No misfortune or disease befalls a Muslim, no worry or grief or harm or distress – not even a thorn that pricks him – but Allah will expiate for some of his sins because of that."
"Every passing moment is a chance to turn it all around"