I have this problem too sometimes. I actually asked a similar question about a month ago if you want to check it out and see if anyone's answers to that help you: How many books do authors tend to write at once?
What I do is I write down a short summary of each idea just so I know more or less what is going on in the story. From there, I could simply choose one and expand on it. But don't let go of an idea just because you're working on something else. You'll end up regretting that.
You could also divide everything into smaller portions. Work on the world building for Project 1 on Mondays, character development for Project 2 on Tuesdays, and outlining for Project 3 on Wednesdays, so you don't have to do everything all at once.
However, the problem hits when I want to write more than one story at once. I found that I used to work on more than one novel at the same time, but never more than three. Now that I'm older, I usually just work on one at a time. Maybe two, if I have the motivation.
I believe it's entirely upon personal choice--and the amount of time and patience you have on your hands--if you want to work on more than one project at once. The reason why I choose to only do one at a time is because I can give my undivided attention to that project and make sure I don't skimp out on it. But some writers may benefit off of writing more than one novel at once. For example, if you're working on two projects, sometimes an idea for Project 1 might actually fit better in Project 2, so therefore it ends up in Project 2.
I find writing groups very helpful, especially when I need to work little things out or are unsure about what to do. Unfortunately, I don't know any writing groups other than Writing SE, but RE Lavender did recommend Wattpad and I heard that one is very popular. Be careful, though, as your uncopyrighted ideas can be stolen.
It's always good, in my opinion, to be inspired by other writers to write your book. Motivation in any form is essential when working on something as intimidating and needs as much dedication as writing a book.
I hope this helps!