Even in love, you might be unhappy. Maybe that's because of the relationship. Maybe that's because of something else. Either way, it can be really, really hard to open up to your signification other about this fact if you're worried it will cause you more unhappiness.
So, how can you do it?
1. Say it clearly: "I'm unhappy."
They might panic for a moment, assuming its because of them, or get defensive, but they will understand that you are serious from the get-go.
2. Explain what you think is making you unhappy.
If you don't know what it is, say that. If you do, try to explain calmly.
3. Don't put blame the unhappiness on them.
Even if they are part of the problem, it's never all one persons fault when problems that cause unhappiness to occur. You might say "ever since you did _____, I just haven't been able to shake this unhappy feeling, so I think it's still bothering me, and I'd like to talk about it more."
4. Communicate, Communicate, Communicate!!!
It comes down to this; you have to say it! If you really have a hard time speaking about it, write a letter. It's not the best option, but it can be a good opener to a real conversation. Write down and explain that you are having a hard time talking about it, but that you want to, if they're ready.