Speaking from experience when you learn online it requires a lot more motivation, especially if you don't have live classes. I did GCSE science and 3 A levels online, along with now having most of my uni course moved online. So I have plenty of experience in managing my time!
The main thing I would say is set a schedule and stick to it! Not always easy I know, I don't always do this with specific times of day, that I will do something, but I might commit that I am going to do 3 hours of studying 5 days a week. That is pretty much what got me through my A levels.
Taking breaks is an important one. If you break things down into smaller chunks of work it can feel more manageable. Then in between each chunk take a short break before coming back to do the next section. For example instead of dreading writing that essay, break it down.
1. First brain storm some ideas,
2. Your next step would be to do some research and make notes,
3. After than you can write an essay plan,
4. Then you can write the body of your essay,
5. Before adding in the introduction and conclusion,
6. After that all you need to do is proof read and spell check.
Before you know it your essay is done and you aren't staring at a blank sheet of paper. If you did each step in 30 minute chunks (depending on your essay length), you could then have spent your target of 3 hours studying and given your self plenty of breaks.
If your concentration isn't there or your tired, don't force yourself to study. Trying to study when your head isn't with it is counter-productive. It will take you longer to learn what you need to, and when completing tasks you are more likely to make errors. What I would suggest is have a rest or go for a walk and come back to your studying in an hour or so.
In short plan out your time so you how much time you should be studying, take regular breaks and break tasks down.