Motivation comes and goes.  Some days you can't wait to get to an instrument, and other days it may feel like a chore.  But if you do it anyway, then you have your determination and willpower online and running.  Expect to *feel* motivated every day and you will likely be disappointed in yourself.  Just do at least your basics even on days when you aren't feeling motivated and try to avoid taking more than a couple of days off a month, and you'll probably be fine.

Music and playing an instrument *are* addictive.  But it is a far more positive addiction than some out there.  Recognize that and make use of it.  The first thing to do is to make it a habit.  Something where it is a part of every day.  The next thing to do is to depend on it.  Play when you are angry, sad, bored, happy, and try to see where it helps with each of those states and whatever else you can think of.  Depressed?  Cool.  Play to make the world go away for a while.  Go through those same stages with alcohol or a substance or something like gambling or porn and you will most likely end up with problems within a few years.  Do it with something like music, though, and it will become such an integral part of your life that you will find time to play and listen every day and you will get to playing better.  

Revenge can also be an excellent motivator.  When I was in my teens, I'd sometimes practice/play for hours a day because some people had told me I should just give up.  I got angry enough about it that there was no way I was going to give up and let them be right.  LOL  Years later, when I had gone through a few years where I'd lost some of my fire, I took a class on recording.  I was in my late 30s, and my classmates were fresh out of high school.  Some of them found my age amusing and one said something like "Hey pops.  Aren't you a bit old for this?"  My blood started boiling and I made a mental note of the person and their friends who snickered.. And I smiled and said "Don't call me 'pops'.  My name is Daniel.  Pleased to meet you."  And for the rest of the semester, I made it a point to have fun kicking their butts in any way possible. LOL  Inside of a couple weeks, I'd put together a home project studio adequate to be able to do all the classwork and projects, so I didn't need to worry about getting scheduled studio time.  In fact, I gave nearly all of my scheduled studio time away to those classmates, and was generally pleasant and helpful.  It made kicking their butts all the more fun, and added to my personal quiet amusement when they had to get time extensions for finish projects while I did extra projects just for the hell of it, to make sure they never brought anything to class where I didn't have something better I'd just finished.  LOL  It was great fun.  I bore them no real ill-will, though, I was just getting in some paybacks.  'Revenge' and showing up someone who annoyed you can be good fun as well as a great motivator.  You don't have to be mean or nasty about it, it can be even sweeter if you are supportive and cool.  But it can be very effective, and I recommend working on learning how to use it when someone manages to tick you off.  LOL

Another thing I find useful is to listen to music.  I don't mean just leaving it playing in the background while doing something else.  Actually listen to it, hunt up works by players who can either inspire you or amaze you, put on headphones and put other distractions on hold and just listen.  Try for at least an hour a day, settle for half an hour if it is all you can manage.  But if you aren't a listener, then you shut off one of the easiest ways to get new musical ideas to use, and one of the ways to keep yourself motivated.