Next year is my last chance for IOI and I wanna give my best to get there. I'm facing a few issues and want to ask for help.

1. I'm very bad at reasoning and proving basic mathematical problems/claims

I face a very hard time when facing mathematical problems regarding number theory and mathematics and their proofs. The only proof I know about is PROOF BY AC. other than that, I don't know anything but sometimes it isn't good. I think I need to develop it on mathematics side and if someone knows some sort of website like Codeforces where I can practice my proving skills a.k.a. basic practice for IMO please tell in comments. My goal is to NOT go to IMO but I think that would help a lot.

2. Need beginner friendly resource for OI practice where I can practice problems in ascending order of difficutly

My current practice rating on codeforces is 1600. I'm planning to solve the entire first page of problemset of 1600(in ascending order). I also wanna grind OI problems because they're quite different from cf problems. The only website I know is qoj.ac on which I find the problems very hard so I'd like if someone knows any website/cf group from where I can start from easy OI problems, please let me know in comments.

Additional info: for some reason I find USACO bronze very hard and I couldn't even solve a single problem in US Open so that's why I don't usually practice there.

I'm very serious about my dedication. Even though I have like 7-8 months for next year's IOI team selection, I'm ready to practice with full zeal and zest.

Thank you!