I agree with this, even though it's probably not what you want to hear.
Motivation is a great thing to get you started, but it tends to wane pretty quickly, so it's good to build in a routine and just stick to it. For example I book and pay for classes in advance, so if I don't go then I lose money and feel guilty - works for me!
Also, try to find and activity you enjoy (or at least don't hate) that way it will be easier to convince yourself to do it. I hate running, so I don't do it, it would be a nightmare to try and convince myself to run all the time, I would be miserable - so I do something else.
I never thought I would be a person who enjoyed working out, but I started doing Reformer Pilates a few months ago and I love it. I do still have days where I'm tired, and would rather sit on the couch and eat cereal - but I go anyway because I know I'll feel better afterwards. And I'm starting to feel stronger which I love. I know it's something I will stick to as I actually look forward to it some days.
I think when you are someone who is unfit( as I used to be), it's easy to convince yourself that fit healthy people have some magical ability to love exercise and just have something you don't - but really, most people who workout regularly don't enjoy every minute, and have days where they would rather do something else - but they like the results, and know it's good for them, so they stick to it.