so the problem is keeping yourself motivated. i happen to hav the same problem too, i resolve by not thinking too hard. It's like not eating a whole piece of chocolate at once even though you know it's there. just nibble bit by bit and your interest can last you a long time. See, if you have enough time, you can write a lot. Also, my cousin writes stories by having a bunch of her friends write like 1 page a day each so that you can have 5 pages a day x 7 days = 35 pages...Hooray, a novel in a month without working hard at all!
If you don't think you have enough motivation then perhaps you know deep down that you don't really like that stuff.
1 other thing: find someone to look up to: if you follow someone else's footsteps then you know that you can continue on easily and you won't give up. For example, I love to play soccer and if you ask any of my classmates how much I have improved, they'll say a crap load! This is because I look up to the varsity captain and practice hard in the street in front of my house for hours. I never stopped and say that i lost interest or anything. If you have an author that you know that writes novels too, then just see how the person does it and follow their way.
Hope this helps ^^