Subconsciously, they mirror others desires, moods, and thoughts. Some people say empaths are highly sensitive, but it goes much deeper than emotions.
Set emotional boundaries.

However, it becomes troublesome when the relationship goes on and you start feeling overwhelmed with your significant others emotions. While this is completely normal to go through, boundaries must be set in order to keep your relationship in good shape.
As wonderful as it is to be connected with someone, it is also a pain. No one wants to deal with other peoples "stuff." Everyone carries some baggage around, and as empaths, we pick up the baggage for them. You need to practice how to say "no" to those who come to you with their problems every day. Your mental health will zig zag across the room if you continue trying to fix others' problems, and half the time, you won't be able to fix them.
Setting boundaries for yourself will help this. It might be hard at first, but please make sure you never take on more than you can handle. Sometimes, the best thing you can do for yourself to set boundaries is sit at home and do absolutely nothing. Clear your mind, and ignore the world for a day. Yoga and meditation is a great way for empaths to clear their head.
Don't compromise yourself.

If you're an empath and you hear these things daily, please understand that there is nothing you can do to fix this. Don't start thinking to yourself "If I just cared less" or "If I wasn't so emotional this wouldn't happen." You are unique. You are special, and it's great that you're so caring and empathetic.
The partner you're with doesn't understand how your mind works, and that's okay. What isn't okay is when they say hurtful things and try to make you feel bad about yourself. Never compromise who/how you are for the sake of someone else.
Leave if you need to.

The amount of emotions you'll feel when you stand up for yourself will almost be too hard to handle. However, think of how amazing you'll feel once you let go of the one person who is making you feel awful about yourself.
A partner is supposed to make you feel special. They're supposed to lift your spirits when you're down, and soar with you when you're at your best. When they're unsure of their emotions, it's hard to be sure of your own.
If you don't feel like you're on top of the world with your partner, why are you with them? Remember, a relationship doesn't have to be physical to be abusive. Mental abuse is just as bad, if not worse at times.
If you're unsure of whether or not you're an empath, check out this test!