I have being going out with this girl for about five years now. Just recently she got pregnant with another guy. She confessed to me that she doesn't love that guy and does not believe she can be happy with him. I later told her if the other guy does not take up his responsibility for the child, I will. But I am confused whether what I am doing is right or wrong. As an aside, I have this strong feeling that I shouldn't let her go; otherwise, she is going to suffer. What do I do? I only want to know if I am doing the right thing or not.
The decision to marry this girl is ultimately your own. But I would like to point out some concerns.
She was having sex with another guy while you thought she was going out with you. I don't know if you two have been behaving yourselves or not, but notice that she has no concept of morality when it comes to sex. She doesn't state that what she did was wrong. Thus, the probability that she would remain faithful to you after marriage is very low. You will likely be haunted by doubts during your marriage.
Second, taking her at her word, she was having sex with a guy she doesn't love. This is not the type of attitude most guys want in a wife. With the absence of a moral desire not to have sex outside of marriage, she also doesn't use relationships for boundaries to behavior.
Third, she indicates that she doesn't find happiness in herself, but expects external events and other people to make her happy. That indicates that she will not find contentment in marriage easily.
What you offer to do is not wrong, but you are desiring to rescue a woman. It appeals to the male ego, but you need to realize that a damsel in distress is often in that position because of her choices. Unless she learns to make better choices, she will repeatedly need rescuing.