Can I discard a ticket if it can't be completed?
There is no way to get rid of a destination card once you already have it.

Destination cards count as negative points against your score if you do not complete them for a reason (think of it thematically - you are playing as a company who has taken on a contract to connect two specific cities by train, and if your company doesn't finish the contract there are penalties), and this is part of the consideration you have to make when drawing 3 and keeping 2 or 3 at the start of the game, or drawing 3 and keeping 1, 2 or 3 later in the game.

You have to strategically choose which one(s) you have drawn that you think are possible to complete, taking into account how likely it is for other player's actions to block you.

Sometimes you may be able to build around a more direct route that has been blocked by other players (especially in early- to mid-game, but less likely during the end of the game), and sometimes it is even worth your while to not take the most direct route, especially where 2 or more of your destination cards include nearby locations.