Your head might be spinning, but you're certain that having a child is not the right move here. For most people, having a baby is the biggest decision they ever make. It becomes an even bigger decision when the pregnancy happens by accident, with more pressure on the choice - and tougher consequences.
There’s really nothing that will make it easier, but for heaven’s sake be honest with the mother about whether you do or don’t want a baby. Don’t live to regret not having spoken your mind at the time. If you feel you have good reasons not to become a father, now is the time to share them.
That said, it is the woman’s right to choose since the baby is in her body. If you decide to go ahead with the pregnancy, she’s the one who will have to carry it to term and give birth. If you decide to ask her to terminate, the experience will be much more traumatic for her than for you. She may not want to have a termination. This is her right, even if you feel it's unfair, or she's doing it to spite you. In all honesty, no woman would ever have a child out of pettiness - it's a not a decision that she will take lightly, and chances are she's turned things over in her mind at least as many times as you have.
Putting the baby up for adoption puts her through birth and separation. Whilst you both have to live with whatever decision you take, the reality is that the consequences are highly likely to fall more heavily on her. That’s why the law gives her the final say and why you should respect it.
Whatever your own views, once you've made them clear, give her the space to actually make a decision. Don't pressure her, and whatever happens you'll be in a better place.