Could you tell only your mother? I would feel exactly as you do about upsetting your father right now. You shouldn't have to change your way of life right away, because the baby is still at least 6 months away from being born.
Look, you didn't do anything wrong, here. Your g/f did. Even after she got pregnant, she did not have to keep the baby. Half of all women will have had at least one abortion in their lifetime. It is the most common medical procedure done in this country. Many teens who got pregnant on purpose, when asked why they don't terminate the pregnancy, will say, "well it's not my baby's fault and it wouldn't be fair."
That is just an excuse because they don't want to admit that they WANT to be pregnant.
If your g/f pressures you to tell your father before you think he is healthy enough to hear about the pregnancy, if I were you, I would tell her that she chose this particular time to get pregnant, not you. If it had been up to you, you would have waited until you finished school and your father was in better health.