Yeah. You are probably depressed. That lack of motivation is not only trying to study, It seems to be around all your regular activities. With that said, you don't have a study motivational problem as a particular thing. Motivation is not something that you have as it was air, blood or neurotransmitters, motivation is a result of your desire of something. In my case I study all day, I read and do my researches. I'm preparing myself for a PhD right now. I been studying 7 years more that you and I want to study for 5 more years. if you ask me "Why" the answer is really simple. It is because I love my knowledge field. And it is not something I discover since high school, I started being interested in this field one year before I did my master degree. Beside this, I like art, music, I paint for money and I play with my mates in those lowlife bars. I have a strong desire of doing things, I love the feeling of improve myself and to discover new things. I do not like traveling for example, so it is not every new thing I like to discover. I have personal preference about what kind of activities I like to do and discover.
You just need to discover yourself first. Motivation does not work on thing you do not enjoy. If I was forced to travel I will start hating my life, no mater how much I enjoy discovering "new things". That does not work that way. Motivation is a result of your desire. If you don't know and accept yourself you will feel that lack of drive and fire inside you. The only thing I can suggest to you is to find something you will love to discover, to do or research. To discover the feeling of improvement, for example I have that sensation when I make some progress playing guitar, when I finish an album or when a learnt how to build in fortnite (I am a little bit obsessed with that game). Your problem is not about motivation, is a bout finding something to burn you from the inside.