My girlfriend won't stop sighing

So, we've been in a this particular 6 years relationship, living together. We know each other very well.

Besides the good things that happened all these years, she never stopped complaining, not a single day.

She shigh from the morning to the night, talks to herself while alone complaining about situations of the day, complains while in shower, gets frustrated and angry for little things, etc.

I can easily tell that she is not happy, and we talked about this many times face to face, but she says I'm not the problem for her, she is just unhappy with her life, past of her family and unfair things in the world.

She has been doing therapy, but it doesn't seem to help her because she still complains a lot.

I'm really starting to ask myself is this is OK to live with, since I'm more like a peaceful, happy, positive guy.

We're probably getting married in a few months, and I do not know if I will be happy with her, having to deal with this problem every day of my life, is taking the energy out of me.

She has light still that remind me of the better days, and for so I still love her, but I'm not sure if we can be happy together.

Should I cancel all marriage plans or should I try it and see what happens? I mean, I realize there is only one life and this is pushing me to make the best decision on being happy and in peace.
My girlfriend won't stop sighing
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