I think with all motivation-less issues the plan should always be to start small.
I started a year or so ago and it took me a while to find the get-up-and-go spirit and many times now I still find it difficult.
Don't listen to the posters above who advise you to give it up - thats not what you need.
Make your first aims and objectives easy to achieve - a ride around the block, a ride to the lopcal shop or the local pub.
Try and choose a a nice sunny day - nothing worse than being cold and wet!
Is the bike you have suitable for you? Is it worth going to a bike shop and looking at what they have -
Halfords normally have some good deals on bikes.
Is it the saddle? Sore bum is no fun!
Any mates who cycle that could join you on a ride? Local bike club maybe?
Worse thing you can do at this stage is to aim for a 20 mile bike ride and then feel crap when you don't achieve it. Buy a cheap cycle computer and watch your ride mileage build up.
Keep trying - slow at first. Maybe even use a mapping app to log your rides.