motivation is incredibly fickle and personal; it comes and goes based on all kinds of factors that can be hard to control, and how it works is different for everyone, so it's hard to give concrete advice on. if you want to map at a consistent pace and see your projects to the end, you're better off relying on discipline.
there are a lot of different resources regarding discipline around the internet that you may want to look into (try googling "build discipline"), but it basically comes down to creating good habits for yourself. for example, if you can get yourself to map even just a bit every day, it'll get easier over time, and finishing your maps will become less of an ordeal.
it's also good to recognise your goals as a mapper as well as what makes mapping fun for you. working towards goals you care about and having fun with the journey are essential for longevity in any hobby.
for example, I want to be good enough to map any of my favourite songs and end up with something I can enjoy playing and be proud of, so I've put myself in what could even be called an elitist environment which always pushes me to grow. I also like the community aspect of the hobby, so I make a lot of guest diffs and participate in other kinds of projects as well.
your goals are likely going to be different from mine, and they'll probably evolve over time as well. if you have trouble with recognising them, it's good to start by thinking about what made you start mapping in the first place and let your goals evolve from that.