My copy of Pokemon Red used to start fine, but now it never does. I just see the "Nintendo" logo with a bunch of dots above it when I try to play it. The cart makes a rattling sound, and I did get it to work in my GBA SP by bending the cart towards me (while holding the GBA) while turning on the GBA. But if I let go of the cart, the game immediately freezes. The cart's contacts are clean, and both Gameboys work fine. What can I do (if anything)?

Try taking the cartridge apart and look for damage.

I tried to do the thing where you heat up a pen and make a tool for opening these carts, but that didn't work. So I guess I'll just have to get an actual bit to open this. It'll be a while
Ok I got the pen thing to work and the PCB looks fine. I don't see any broken pins or soldering...so I don't really know what to do now.

So i think I know what's up. The bottom part of the contacts on the game are in really bad condition. When I bend the game away from me, the top (good) part of the contacts is making contact with the Gameboy. So I guess I'll have to find sorm way to repair the contacts.

Did you clean the game really well with contact spray and/or rubbing alcohol? I noticed that even though contacts look clean they might still not make good contact.

I cleaned them quite a lot with rubbing alcohol. I'm mostly convinced that the problem lies in the bottom half of the contacts being really scraped up.