If you're worried about jinxing your project, just find something steadfast to motivate you. Who gives a crap what others say? I heard that JK Rowling was rejected around a dozen times before Harry Potter finally broke through, and that story is a reference point for authors now. Don't let sharing your idea with someone else be enough to kick your story underground.
Lying is never a good idea, but maybe you can make it seem like you have another obligation (just something to reduce questioning on why you are alone so much). Or when you're around your friends, make it clear you aren't a loner, be extra engaged, just show them that you're totally good, if you are worried about them being concerned. But I honestly wouldn't worry about it too much.
Just go for it while maintaining a balance. And also - if you really want to take this somewhere, which I'm sure you have the capability to do, why hide from people? You'll have to get it out sometime, right?
Good luck with your story.