I don't know if this helps, but I personally find coding interview practice sites/problems to be extremely boring, difficult, not fun, and frustrating to work on. I'm saying this as a 13-years-of-experience software engineer; I've been called a rock star or ninja or something similar in almost every company and team I've worked in (big and small), and throughout my entire career and the numerous times I've job-hunted, I have never passed a single interview that involved those types of questions. (I've interviewed at both Google and Facebook, both more than once, at different points of my career, with no success) I pretty much always end up at a company where I had a really easy interview and we just talked about my experience and projects. I am (I'd like to think; and I think my coworkers mostly agree) good at everything involved in a software engineering job (building things, architecting things, communication, working well with others), but I absolutely cannot solve those whiteboard interview problems, especially during the interview.

I saw that itamarst posted a link for a list of companies who don't do "whiteboard interviews" type of questions. If you believe you're a good dev and can build stuff and just hate those interviews, I would encourage you to start there and try interviewing at those companies and land your first professional job, and see if you're really cut out for a software engineer role.