Writing Multiple Stories at Once

You know, I always get new ideas for stories, and end up writing more than one at the same time. I really don't know if anyone else has this problem, but it really annoys me. Although I have a system for doing more than one story at once, I really need to find a way to content myself and just focus on one thing. Also, if you're wondering why I'm doing this now, is because I thought of another really good series, that I'm in the process of beginning. Now, you're wondering, "What's his system for writing two stories?" Well, here, I usually monitor how long ago I update stories, why? I don't know, it's a habit. Anyway, I usually just write a chapter for one story, post that, check out which story was latest updated, and read the last chapter I posted and start writing the next one for that. Anyway, I'd love to hear you're guys' solutions to solving this problem, please, HELP ME!