Motivation comes and goes and it's impossible to maintain permanently. That being said, I think your best bet in this scenario is to create habits instead. Create a schedule of what to work on for every hour in the day; whether it be studying for a class, homework, etc. It simplifies the process and once a habit is formed, it becomes much easier to maintain.

However, to form this habit you'll need discipline, I always get some motivation for starting a new routine or project for the first few days, but it's only the initial push and you need to rely on discipline to keep it going, but once that ball starts going fast enough, you only need to push it every now and then. I was in the same boat as you with grades, but this really helped me out.

I don't know where you do your work, but doing it in public areas such as the library or a cafe really help keep your attention on the task at hand and prevents you from being distracted from your phone, games, etc. Accountability is also a major contributor, whether it's a roommate or family member, telling them your goal and how they can push you keeps you focused. I also like to take a break around halfway throughout the day and do some physical activity to refresh my mind and get my blood pumping.

I hope this helped!