I hate how when you accidentally press backspace, then forward again, and wham, the text is gone. I spent at least 45 minutes writing all that.
Well anyway, going to make this short since I've already spent 45 writing what you have read. Working on several simultaneous stories does not affect all writing quality to my knowledge, it is only a matter of opinion. Do not succumb to JJJ's professional writer's criticism, for one, this is a story game site who some exploit as a hobby and not a recurrent job where you have to make quality count for the deadline you're trying to meet so you can actually feed yourself and have a roof over your head. Second, not all professional writers focus only on one story at a time because not every person writes in the same way or feel the same way or think the same way. Some just are better while multi-tasking and some not so much, therefore, do not take everyone's criticism to the point where you try writing with your left hand instead of the right where most of your work usually gets done. However, it also wouldn't hurt to try having one story and see if it works for you. Look at the quality, is it better or worse? If better, good for you, if worse, ignore the peer pressure.