This is super unfortunate that this is how I came across your stories only to find out that you are leaving. :/ Of course, no one can make you stay, but I hope to read your stories elsewhere if you really choose not to stay with Mibba. Best of luck to you and making your decision.
I'm really sorry that you feel this way. As a story editor myself, I feel it's worth mentioning that we actually don't touch the stories themselves - we only have access to basic things that we are told to correct, like your title, short and long summary, and things like the rating, type, etc.
Being told that you need to fix your grammar is by no means a way of telling you that you're a bad writer. I understand people saying that Mibba shouldn't be so strict, but I have always (and I mean always, even long before I was a story editor) seen the "have correct grammar" rule as a way of improving people as writers.
I am very sorry to see you go, and sorry to hear that you won't be finishing your stories. =( That breaks my heart.