I intubate standing up. Yes, into the sink. And I do keep it very clean! You may try running very cold water over the tube. Makes it stiff. Some take an ice cube to the bathroom, and rub it over the cath to make it stiffer. Use lots of lube.
Also, if the tube kinks at one of the holes, throw that tube away and use a new one. The old tube will just keep kinking in the same place.