Thats sad..you better stay with her....I
But if your not happy you should of never knocked her up.
Sounds more like you're feeling stuck and in a mid-life crisis sort of deal. If you really felt this way you would not have agreed to have a child with her. You have a son/daughter to think about now. You may not have the Heidi Klum of girls but if she was enough for you for 15 years.. you need to stick around. You have started a life together.
Your emotions seem conflicting. You say you love her but you don't. Yet you stayed with her for 15 years and no matter how pressured, you still agreed to have a baby. A lot of what you wrote implies that you feel forced into the situation and there was nothing else you could do but comply with her/life's requests.
The situation is complex now since she is pregnant, living with and financially reliant on you. While you find yourself drowning in helplessness, you found someone who seems to take your troubles away. I would suggest your best bet right now is not get distracted by the other person, try to work on the situation you have now. Take the situation in your own hands instead of feeling forced, work on it, get professional therapy and guidance if you have to. Find out what is the root of the problem. If all else fails, then you deserve to make that final decision.