I slept with a single girl who as being single was a nidda. Am i chayav karies? What does this mean for me now? Is it possible to rectify? I obviously regret what happened and I’m determined to repeat it ever again. Lastly is this aveira forgiven like all aveiros are once someone gets married on the day of the wedding?
You are correct. Since she was a nidda then the aveiro is one of kares, which is obviously a very serious Aveiro. However, H-shem in his infinite kindness gave us a way to rectify the areiro and we can do teshuva for this and all other aveiros. There are three essential parts to teshuva. The first one is Charata- remorse, that you genuinely feel bad that went against the commandment of H-shem and that you feel bad about it. The fact that you wrote this letter shows that you already did this part. The next part is viduy- to confess the sin verbally to H-shem (not to anyone else) and say, “H-shem I sinned to you by doing … and I am sorry about it”. The third step is to accept upon yourself that you will NEVER go back to this sin again. It has to be a strong commitment, and you should do something to help yourself ensure that indeed you will not fall into such a situation again. I don’t know you or your situation, but you have to take positive steps to ensure that you don’t come close to the original nisayon that brought you to this sin.
In addition to this, since this is a big sin, I would suggest that you also do the following. There is a posuk in Mishlei 16 that says, בְּחֶסֶד וֶאֱמֶת יכופר עון “With truth and kindness a sin is atoned”. Emes is referred to as the torah, which is emes, and chesed is referred doing kindness such as giving tzedakah. Therefore, I would suggest that you increase your torah learning in some way, and that you give some money to tzedakah.
When a person gets married, there is a degree of kapara that he has, but it will not exempt the person from doing correct tesuvah.
May H-shem help you to do a full teshuva.