You're not a loser, you have 8 years of development experience across multiple companies. Nailing the technical interview does not guarantee you get the job. As you mentioned, luck does play a factor and being hard on yourself for not passing an interview is time better spent preparing for the next one.

claude and ChatGpt are your best friends, ask them to act like "the company and the position u r going to interview for" and take ur interview.

HelloInterview has helped me I think. Especially with system design

Practice. Use pramp.com. Do super super simple ones. If you can't do two sum or fizzbuzz or tree traversals with someone watching you, you're not going to have a prayer for harder stuff. Just start SUPER simple. Pracitce a lot.

It should be muscle memory to do the easy stuff. And then it turns out, the 'hard' questions are often just a combination of easy things.