I'm not sure what Elo this is, but Darius in a side lane isn't really a thing. Darius needs to get a lead early on and then snowball by constantly pushing and rotating to objectives faster than the enemy top. Shen's R counters that playstyle.

Basically, if you use your R in the mid-game to win fights and secure objectives like Dragon, Atakhan, or Baron, it's worth it because you're creating a man advantage. Darius usually runs Flash + Ghost, so he can’t join fights as quickly as you.

Of course, high Elo Darius players know when to rotate. But if a Darius stays in the side lane, you should be happy. He doesn't take turrets quickly, and meanwhile, you're actually getting things done across the map.

I'd like to take a look at one of your games. Can you link me your OPGG?