I did something I feel really guilty about

Difficult one this.
First off, I think you're being hard on yourself.
I think it's pretty understandable why you didn't tell the truth.
I actually don't think this is about being caught cheating in any case - its more about the betrayal you feel for not having been honest with your friend, who you clearly value very highly.
It seems to me there's no perfect solution here. Your friendship is at risk whether you tell the truth or not.
Think about your own statement that you wouldn't want to be lied to like that - and maybe start from there.

Ok, it’s not what you’re thinking. But;

I cheated on my physics test— something I have never ever done before, but I had major anxiety the whole weekend over this test coz if I failed it, I would be held back in the year. The invigilator caught me, called the principal and my paper was cancelled. They took me down to her office, called my mum and then let me rewrite the test.
When I got home that afternoon, a guy in my class (we’re really close, more than friends but not really together either) called me and asked what had happened. I lied to him. I lied coz I didn’t want him to know what I had done. I have never lied to him about anything but I was so ashamed of what I had done I just couldn’t tell him.
For the week after that, he was acting kinda weird and distant. Last week, he texted saying “I have to ask. Did you or did you not cheat on that test”. I panicked coz if I said that I did, he would know I lied to him and that I cheated; I said no!

Now it’s eating me alive, I’m spiralling, I’m so scared he’ll find out from somewhere else (no kid knows about it but the teachers might say something)
Idk if I should tell him that I lied to him twice about it and I did actually cheat, or just keep covering this up and live in mortal fear of him finding out. We’re going good rn and I don’t want to mess that up. I know I wouldn’t take kindly to being lied to, and I would think less of anyone that does cheat on a test, so why wouldn’t he. I hate myself for cheating, and I hate myself for lying about it. I feel so guilty and idk what to do

It's important to be honest with your friend about what happened and why you lied. Keeping secrets and lying can often make things worse in the long run. You should tell your friend that you cheated and apologize for lying to them. Be honest about why you did it and how you feel about it now. Remember that everyone makes mistakes, but it's important to take responsibility for our actions and learn from them. If you both value your relationship, then being open and honest with each other is the best way to move forward.