Baruch Hashem, I’m very soon approaching the 9th month of pregnancy. However, it should be noted that by the time we conceived, we weren’t keeping the laws of Mikve, which personally I deeply regret and feel bad about having a Ben Nida.
I wanted to know if it’s possible to repair this Avera and what should I do?
It is nice to see that you are growing in your observance of the mitzvos. May Hashem help you to keep on growing.
Regarding repairing the aveira of not going to the mikvah, it seems that you are doing teshuva on this, by the fact that you are now keeping the laws. Keep it up, and I would advise you to learn the laws of nidah well and this is a way to rectify that aveira.
Regarding the pregnancy, and conceiving when you think you were a nidah. First of all it is possible that in retrospect, that you were not a nidah at the time. It is possible that you went to a large swimming pool, or went swimming in a lake or beach, and although we wouldn’t usually use it as a mikva, but it would still help that the child will not be considered a ben nidah.
Secondly, it is important for you to know that a ben nida does not mean that the child is doomed, it is not true. The child may marry anyone that he wants. Especially if the child will be brought up with a good education, in a religious school, and he will be educated to have good middot, then you don’t have to worry. There are many big people with fine middos whose parents were not religious, such as R’ Akiva, and some of the biggest people in our generation, (without mentioning names). If the child is given a good, religious education you don’t have to worry. The gedolim say that a person does not have to worry about marrying the child of a baal teshuva, because if the child has good middos and got a good education, then obviously this idea does not apply to them.
May Hashem help you that you should both continue to grow, and you should merit to bring up very fine children, who will excel in piety, torah and mitzvos.
Shulchan Aruch E:H 4-13, Igros Moshe Y:D 4- 17 (19), Kehilos Yackov Yevamos 49 regarding shidduchim.