organisation help Watch
I would set goals of accomplishment every week instead. For example, for history, you could set yourself a goal to finish making notes on a chapter on American history, and another goal to do practise questions on a particular textbook page. Similarly, set goals for all the other subjects so that you end up with a continual weekly goals list that you can work through steadily within the time you designate for studying. This means that you won't be stuck doing a subject that you timetabled for yourself in an hour, when you feel like doing another subject.
If timetabling works for you, but it's the discipline aspect that you're struggling with, you can find apps like Study Plan, or Timetable. Personally, I used My Student Life, which helped me plan what I wanted to do and how long I wanted to do it for. It can get a bit fiddly inputting all these times but you get used to it and it's very helpful. Plus, the neat aesthetic of the app gets you in the mood for productivity