I’d say тайно любить кого-то. It’s not important how long, what’s essential is that these feelings are hidden and not reciprocated.

Like please

Быть безответно влюбленным?

Not necessarily. If you look at examples online, you’ll see that we almost never use this expression without the word STILL because it means that at some point you were both together, but the other person moved on, and you continue to have feelings for them. So it’s not so much about the fact that it’s not reciprocated as it is about the fact that you haven’t moved on; that you’re still in love.

ОК. Тогда скажем "быть безнадежно влюбленным". Надежда ушла вперед, а любовь к ней не угасла и все еще тлеет где-то в глубине души аки одинокий факел в кромешной темноте ночи...

Звучит хорошо 👌🏼