I thought Bernie Sanders was the single best chance America has ever had for real change, change that would at least bring it closer to parity with the rest of the so-called first world. When the Democrats spurned Sanders in favour of Biden (with the subsequent support of Hillary Clinton, no less), I was genuinely gobsmacked. I felt like they were trying to lose. I couldn't imagine a worse candidate - if one single candidate had a decent chance of losing to Trump, it was him. And sure, if Biden were standing for the Welsh Assembly or the UK Parliament I'd probably think twice about voting for him too. It sucks that those are your only two realistic choices. The South Park sandwich sketch comes to mind.
But you live in a two-party system. It's wrong and it needs to be reformed, but it's what you've got.
If Trump wins again, the damage not just to America but the rest of the world will be incalculable. Think of it - a Trump unconcerned with winning any votes ever again!
In the past I've scolded people for the idea of 'tactical voting', as I don't generally believe that one should sacrifice their principles for partisan reasons, but in this one instance, I think it's important to do that. Whatever problems Biden has, he can't possibly be as bad as a resurgent Trump with no concerns about being re-elected.
If you genuinely think that your vote won't matter no matter where you put it, go with your gut. But if there's even the slightest possibility that your vote could serve to remove the incumbent president, I would urge you to use it appropriately. For all of our sakes.
Fix the system after the system isn't being dismantled by a lunatic.