Please, sir, there is something I want to discuss with you. I lied to my pastor and since that time I have been thinking about it. I am in a relationship with her daughter. We are planning to marry. I slept with the lady and the church members found out that I am going out with her. they told her mother about it. She asked me a lot of questions about our friendship. She was a little bit aggressive so when she asked whether I slept with her daughter or not. I said no. Since that time I haven't been of myself because I did not tell the truth. I love the lady and don't want to lose the relationship. Please, sir, what do I do?
You managed to pack a lot of problems into a short note.
If you are attending a church that has a woman as its preacher, then you are with a group that is not following the Bible (I Timothy 2:11-15).
It appears you are more upset about lying than that you've committed fornication. Both lying and fornication, if not resolved will keep you out of heaven. "But the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death" (Revelation 21:8). It doesn't matter that you plan to marry this woman. What matters is that you weren't married and had sex with her.
But like the fornication, you justified lying because the mother was pressuring you. That is no excuse for being dishonest. You lied because you were afraid of the consequences if people found out that you have been having sex.
What do you do? Arrange to get married to this woman, assuming she wants to marry you. Stay away from sex until you do get married. And since the mother made choices based on your lies, you and your girlfriend need to talk to her mother together and tell her the truth.