Self-medicating with Modafinil?

It’s super effective (and legal to purchase over the counter where I live), but it gradually loses efficacy, and by the third/second week of a modafinil cycle, tasks become harder, focusing becomes a challenge and I gradually lose all motivation. In the end I just give up and enter potato mode again.

And what I meant by a “modafinil cycle” is that I use it for three weeks or so then stop it for a week. Then I start using it again. My problem is that I have a huge work load recently and it’s getting harder to keep up.

So I’m wondering if anybody else does the same? Am I slowly destroying my brain capacity by doing something dangerous? Do you have any tips or know any alternatives to what I’m doing?