To me, it sounds like you have already identified the major issue. There does not seem to be a structural issue at play here. There is nothing inhibiting you from finding academic success. It is a matter of your own choices and will. The good news here is that you are going to be the author of your own narrative and responsible for your own success.
There are many different approaches to take here. I would suggest two, in particular. The first is a mind set needs to be embraced. You are going to have to "want" success in this upcoming year. You are going to have to motivate yourself to want to do well and to put in the work to maximize the talent you have within you. One way to do this is remind yourself that 8th grade is an important year. Essentially, the primary function of 8th grade is to ensure that you have the necessary habits of academic success that will guide you in high school, when the workload suddenly increases in amount and complexity. If you don't have the necessary habits of a successful student that have been honed and developed throughout an 8th grade year, it becomes much harder for you to "turn it on" in high school where you will be overcome with so much that is different from where you are now. Academic, personal, extra- curricular, and social changes will all be upon you with such force in high school that you are not really going to want to figure out how to "get homework done" or how to "apply yourself." I think that spending your 8th grade year getting these habits down will help you immeasurably for high school. This might be one of your motivating factors in adopting a new mindset for success in 8th grade. You are not really playing for 8th grade as much as you are for what will come afterwards.
The next issue is practical. Essentially, you are now going to have to figure out how to make these habits evident on a nightly basis. Not knowing what your routine is now, it sounds like you will need to develop some good and solid habits that are not there now. For example, it might be in your interest to make sure you have a good study area in your living area set up and devoted only to studies. Minimal distractions should be present, and it should be your area to work. Additionally, starting homework at an appropriate hour will allow you to spend at least thirty to forty minutes on each subject a night. That is a good habit to start developing in 8th grade because it will be needed in high school. Too often, students do work in one subject, and see improvement there, but drops in other subjects. Essentially then, the student becomes a circus juggler, trying to juggle one class over another and soon it becomes too much. This condition can be avoided with devoting about 30 to 40 minutes in each subject, each night.
This will also have to be supplemented with how you are doing in class, as well. You should be demonstrating the same successful habits of academic success in the classroom, as well. Ensuring that you are focused on instruction, asking questions when you are unclear, and doing your best to harness your academic potential towards academic success, even when your classmates might not be doing that, is going to be critical. Once again, this feeds into the idea of what will be expected of you in high school and using 8th grade as the ground for which these ideas can be practiced and refined will help to make your high school experience more successful.