It is really important to constantly work with this kind of stuff. The most people who say that they are bad at maths haven't tried hard. If you can't solve a problem, don't give up and try another similiar one after someone has shown you how to solve the first one. In the 8th grade you typically won't need to have the greatest own idea's to solve the problems presented to you since the problems are very replicative. So I can give you the following tips:
Learn the important formulas by hard. There are very few you need to know but if you don't know them you can't solve any problems.
Get an idea of what the formulas say and how to apply them. That is almost as important as knowing them in the first place.
Observe the problem types and learn how to solve them. There will be very few problem types and if you can recognize them as such you will have an easy time solving them if you know how to solve that specific type.
Don't give up and don't fall behind. There a two very important things in math: The first is practice (!) and the second is that you keep up with the new stuff you learn. In maths everything is building up on the things you learned before. So if you fall behind you will get much greater problems in the future.
Chill down. You are in the 8th grade and there are much bigger problems in life than 8th grade maths. Don't pressure yourself too hard. The fact that you inform yourself before the 8th grade is very good in itself. You know what you will be facing and that's more than the most kids at your age know.
I'm sure you will get this, greetings from a german master student :)