Ah yes! All to similar to most of my educational years.

First off, is trouble getting motivated because going to class is just not something you want to do (aka maybe you find it to be boring, etc.), or do you think it may be an underlining cause of something else?

If you're having to do what you'd rather not, then maybe you can find something to help get you motivated. For example, I'm terrible about getting out of bed in the morning, so when I set my alarm the night before I make sure to place my phone just far enough away from my bed so that I physically have to get up to turn it off. Maybe find something to do before class that is fun, or relaxes you so that you aren't as nervous. If you have time, maybe take a walk or exercise. Sometimes once we are up and moving its easier to do other things.

The reason I ask if it may be a small part of something else is because you mentioned you feel as if something bad is going to happen if you go to class. May I ask what do you feel will happen that is so bad? Could it be something like being around other people, or in another place, or do you feel panicked like this sudden dread has come over you? Sometimes these could be symptoms of an underlying anxiety or panic disorder. Since you say it's fine once you get there, there are helpful ways you can combat stress and anxiety (though I apologize for I cannot think of any off the top of my head right now). However, it keeps being a problem, or becomes too overwhelming, you may benefit from some professional help. Now, that doesn't necessarily mean a diagnosis, or medication, but talking to a school counselor, or a therapist can be a big help.

I hope you can find some relief soon. I was (and in some ways still am) dealing with a similar situation since middle school, and it can be extremely stressful, and frustraring. All the luck.