Being with a toxic person can be physically, as well as emotionally, depleting. I have watched people deteriorate physically after being in an unhealthy relationship for too long. Negative people can suck the life right out of you, which is why they are sometimes called “emotional vampires”.
There is more than one kind of toxic behaviour: people can be passive-aggressive, where they never follow through on what they say they will do. Other people can be overtly mean and even abusive. And, of course, covert behaviours abound when you are with someone who is always trying to keep you in your place, no matter what the cost.
These behaviours can take the form of emotional abuse, sarcasm, dishonesty, and name-calling. If the person with whom you are involved is behaving in a toxic manner toward you, you need to get help or get out.
It may be true that this person was nice in the beginning of your relationship. In toxic relationships, what often happens is that the negative behaviours come out over time, and it can take a while to see that this person you have loved is becoming someone whom you no longer want or need to be around.
You may have felt a growing distance, or you may never have noticed a problem until he or she did something egregious. It can be a feeling thing, where you just wake up one morning and look at this person lying next to you and wonder to yourself, why am I here with someone who is hurting me?
Allowing yourself to experience a barrage of meanness from someone who says they care about you is self-defeating. You may tell yourself that he or she is “just upset” and will calm down. After a year or more of this kind of treatment, however, you need to see that things will only get worse if left alone.
I am a champion of relationships. I believe that a good relationship can change your life for the better. But if you are being taken down by the words and deeds of a romantic partner, it is time to reconsider what you’re doing in the relationship. If you do not protect yourself, you can become ill, numb, and disconnected, which will make dealing with every aspect of life more difficult.
Taking a positive stance and creating strong boundaries, where you do not accept toxic behaviours in your life, is an empowering place to start. 
It’s important to tell your partner what you are experiencing and how you are feeling. If he or she does not want to heal this broken part of your relationship, you have your answer. 
If your partner is willing to do the work necessary to get you back on a healthy track, then you should start immediately.
You can watch some videos or even read a book together. Just remember that communication is the most important thing, and you need to get talking, so the healing can begin. This can be resolved; start slow and get professional help if necessary.

*Dr Barton Goldsmith, a psychotherapist in Westlake Village, California, is the author of The Happy Couple: How to Make Happiness a Habit One Little Loving Thing at a Time. Follow his daily insights on Twitter at @BartonGoldsmith, or e-mail him at Barton@bartongoldsmith.com