I have started and ended a LOT of hobbies in the past. When I started these hobbies, they were heavily dominating my daily thinking and sometimes even led to an obsessive analysis over said activities. Once I got excited about something, I never had any difficulties in motivating myself to do these things. For example, when I decided to practice playing guitar, my fingers would be so sore from all the practising the next day that I couldn't practice any more without taking some painkillers.
But after a while, I lost interest in these hobbies, decided to skip some practise days ("I'll just do twice the amount tomorrow") and eventually dropped them altogether. One day I was browsing through the king of all procrastination sites, Reddit, and ran into this short article on discipline. It is a good take on motivation versus discipline, and goes on explaining that
"Motivation, broadly speaking, operates on the erroneous assumption that a particular mental or emotional state is necessary to complete a task."
Basically, if you wait until you're ready to do the task, you're doing it all wrong. You can check out motivational posters all day on the Internet and wait for that right feeling, or you can turn on the robot mode and do the task because you simply have to. This is discipline. There simply isn't a way to motivate yourself into doing the most tedious stuff like washing the dishes or working a 12 hour shift in the plastic bag factory. Here's a definition of discipline from the article:
"Discipline, by contrast, separates outwards functioning from moods and feelings and thereby ironically circumvents the problem by consistently improving them."
Chasing for that motivational push is only making you feel like you should do things when you feel like doing them or if the end result or the goal is something to be desired for. Of course this doesn't apply for all (well, most) necessary tasks in your life. Motivation often gives you a temporary dopamine high from success which can be compared to the feeling you get from alcohol, drugs, fast food, etc. - you feel great for a while but after the feeling fades, you start looking for that next 'motivation high', be it from another motivational poster or a book/blog post about motivation.
Of course motivation has it's uses, for example in gaining energy to a particular purpose or task. It can help you get that mental energy for a presentation (for this I can warmly recommend this TED talk) or for a competition, both being very specific contexts. But motivation is very, very bad philosophy for day-to-day activities such as going to the gym or reading a text book for an exam. Therefore:
"Discipline, in short, is a system, whereas motivation is analogous to goals. There is a symmetry. Discipline is more or less self-perpetuating and constant, whereas motivation is a bursty kind of thing."
Cultivation of discipline comes through habits, of which you can read more about in this blog post.