It’s likely not you. As this piece argues, Learnable Programming, there are limits to the approaches that online sites have which many can get by, but some cannot. If you are in the latter group, give this piece a read. Afterwards you might think: aha! That’s why I couldn’t get it!
With technology, if you don’t understand something, don’t assume the problem lies with you. It may, but most of the time, the problem lies with the technology.