I have to admit that I was bitter for several months after I decided to leave Islam.I felt like I had been living in a huge lie my whole life,that I had spent the best years of my life suffocating trying to fit into the system. It was like wearing a straightjacket, not being able to be myself, and in fact doubting my own mental processes as if everyone else was right then I had to be wrong. I was angry and bitter at society's injustices that i saw everywhere that were either caused or perpetuated by Islam, and at the self-righteousness and stupidity of people trying to follow the rules. I felt that it was too late for me now, and that i had been in some way crippled, my personality fundamentally flawed by the attempt to fit into the muslim world, that I would never be able to integrate, fit in with any society, never be able to function normally.
I couldnt really say who my bitterness was directed at. At society in general, at my parents, at all my friends and acquaintances, at religious scholars, at the prophet mohammed, at the sahaba, at god whether or not he existed...
My bitterness faded when I stopped seing things in black and white and realised that the founders of Islam and the people who perpetuate it today did actually ( I believe) have good motives, thought they went wrong in a major way. My view on religion now is an extension of my view on human beings.All of human history is flawed because we as human beings are flawed. Its best to accept that.
It also faded as my life began to be better, Im not sure which came first (chicken or egg ?) but eventually I started to be happier and started to enjoy life, open up and regain faith in myself... Being able to get away and live far from my previous heavily Islamic environment helped.
I think its completely normal to be bitter initially but after a while its just no use, it eats away at you, its best to let it go and move on.
I dont think its right for anyone to be angry at themselves for having been duped though. It doesnt contribute in any positive way, it just lays unnecessary guilt. Islam like any other life experience is a stage in our development. Its my history and life experiences that made me who i am today (whether or not that is a good thing is another matter

)
Im not sure about gratitude though... why be grateful? Sure I've generally had a great life, Im grateful for that, but how much of that is directly due to Islam? I would say its bitterness versus acceptance.