Hey Meredith, so I wouldn't worry about your diagnostic test. The similarity in your scores probably assumes there was a trick question, or something you misread. If you didn't KNOW the information, there'd probably be a greater deviation. Irregardless of the reasoning, I wouldn't worry about it.
As far as physics goes, it's very important to understand the beginning information by any means necessary. I am good with numbers, and physics. However, I will tell you that physics is probably one of the most difficult courses in high school (harder than calculus -- despite not needing it, presuming yours is algebra based since your in algebra). In fact, most people will tell you that physics is one of the hardest classes they ever took.
I highly recommend using KhanAcademy for physics along side of
Doc Schuster on YouTube. Doc Schuster will be an invaluable resource in intuitively understanding what's happening. I'm not sure what part your on, but presuming Kinematics, the easiest way to think about this is just substituting values for the variables in your kinematic equations, and solving for what you DON'T know. You also need to establish a coordinate system. Typically in algebra based this simply means having an x & y axis, where anything that moves UP is +y and down is -y, and right is +x and left is -x. This is critical and may be a huge source of your problems.
The last thing I recommend is getting a Schaums book on physics. It gives example of EVERY physics problem you will encounter and HOW to solve the majority of them (step by step instructions). Physics isn't something where you can just go to class, read the text, and get a good grade. You HAVE to do problems. And you HAVE to do them more than once. Typically, the first time you attempt a problem, you'll get it wrong. I remember my first physics course. I'd spend hours on a problem, getting it wrong, time and time again. Eventually, I'd get it write. And as I started to get them right, I started to understand HOW the problems worked.
The thing with physics is it doesn't work like math. There isn't a simple algorithm you can use to solve the problem. You have to know how to model the problem since every problem is unique. The only way to do this is to do problems until you understand it.
Physics is a hard ass class, that requires a lot of work. Don't expect to get it first try, second try, or even the third try. It's going to take time and dedication. The most important thing you can do for yourself is understand Newton's laws. After that, physics will make sense. Newton's laws appear easy, but applying them is difficult. This may take you all semester to grasp.
Lastly, if worse comes to worst, talk to your professor. I'm not sure how you're taking Physics without having trigonometry (depending on the physics your doing). Are you taking conceptual physics or physics?
Anyways, good luck. I'd also like to mention, try not to think that you 'suck' at school. Some people just have to work harder than others. I did terrible in highschool. Now I'm going to a good university in one of the most difficult majors. The difference is I had to work a lot harder than my counterparts. But let me tell you something. Physics is to date one of the hardest courses I've ever taken. So don't feel discouraged. It's a huge hurdle to get over, but once you get over it, you'll see that what you were stressing about wasn't so bad after all. But until you get over that hurdle, it'll feel like hell.