How should I break up?

I have been into a relationship for four months. After that time I realized that, even when I feel very confortable with the girl I am with, she is very different than the person I wanted to spend my days in, for example, a marriage.

She is very happy been with me, but the question is I am not. On the other hand it is so heart breaking to me telling her I would rather prefer to stay away for our current relationship. I was dumped in the past and it was the worst time on my life, and I think it would be like the same to her if I dump her.

So what are your thoughts on this? Thanks in advance.
How should I break up?
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