I am a 26 female I have slept with a married man that is 57. I told him it was wrong when he asked me. I need to know how to ask God for forgiveness and to get through this. He also knows that he has sin and we agreed that we were not going to do this again. The problem I am having now is that I miss him and it feels like I need him but I know it's wrong. Please answer me.
Often we do things we know are wrong before we do them. That seems to be part of the nature of man. However, that is not God's will, as you obviously are aware.
I will answer your second question first. How do you deal with your feelings for this man? Hard as it may be, it is possible to forget him, and each time you find yourself rejecting the idea of being with him will make it easier to do the next time. Remember the promise of God in 1 Corinthians 10:13. "There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it." The problem comes in finding the way of escape God provides. One way in your case might be to avoid all contact with the man in question, if possible.
Another would be to fill your life with other things. If you are too busy to think of this man, eventually you will stop thinking of him. It would perhaps fall under the idea expressed in Philippians 4:8, "Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things."
A third possibility, and you may need to look for others beyond these, is that God will provide you with a single man you can love and who will love you. But as long as you are emotionally involved with this other man, even if not physically, you may not see the man God has for you. Spend more time looking for the man that can be yours, rather than the man who is someone else's. Among the commandments to the Jews was one not to covet someone else's spouse, even if that spouse is willing.
Now, as to how to ask God for forgiveness, I will provide specific scriptures with a minimum of comment. There are several things you need to do to obtain forgiveness for this and all your sins.
One of those requirements you clearly have met. One must believe in God and in Jesus the Christ as the sacrifice for sin. "He that believeth and is baptized shall be saved; but he that believeth not shall be damned." (Mark 16:16) "For ye are all the children of God by faith in Christ Jesus." (Galatians 3:26)
Another is that you repent of your sins. Simply put, this means both feeling sorry for them (which you do) and determining not to do them again. "Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost." (Acts 2:38) "And the times of this ignorance God winked at; but now commandeth all men every where to repent." (Acts 17:30)
Scripture also indicates we must confess publicly that we believe in Christ. "Whosoever therefore shall confess me before men, him will I confess also before my Father which is in heaven." (Matthew 10:32) "For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation." (Romans 10:10) This is not confession of sins, but a public acknowledgement of one's belief in Jesus as Savior.
A fourth requirement for forgiveness is immersion in water, commonly called baptism. It doesn't matter who baptizes you, whether preacher, any Christian, or even any non-Christian. What is important in baptism is you, and that you are doing it for its intended purpose, the removal of sin. In the passages already given baptism was mentioned as a requirement in Mark 16:16 (in the point about faith) and Acts 2:38 (in the point about repentance). Here are some other passages that say it is essential for forgiveness of sins (salvation). "And now why tarriest thou? arise, and be baptized, and wash away thy sins, calling on the name of the Lord." (Acts 22:16) "Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death? Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life." (Romans 6:3-4) "When once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water. The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ." (1 Peter 3:20-21)
Meeting these requirements will obtain forgiveness of sins, all sins, even the sin which is so bothering you right now. However, it doesn't guarantee you will not sin again. If you have previously met God's requirements for forgiveness (faith, repentance, confession, and baptism), but then committed this or any other sin, God has provided a remedy. He does not require that you be baptized again. He just requires your acknowledgement of sin and request for forgiveness. "Repent therefore of this thy wickedness, and pray God, if perhaps the thought of thine heart may be forgiven thee." (Acts 8:22) It is important to point out that this was a person who had already been baptized for forgiveness of his sins.
Perhaps this answer has been a little long, but I wanted to answer as completely as possible. This was in part because many people will give only a partial answer, and would have told you that all it takes for forgiveness is a simple prayer. I hope that this answer will help you.
Thanks for responding to my question. The problem I am having now is that he called me. I have asked God for forgiveness, but I don't think he had asked because he wants to see me again and I do not know how to tell him no.
The easiest way to tell him "no" is just that. Explain that you know it is wrong, and you want to quit. Tell him that if he chooses to keep sinning, it will have to be with someone else, because you want to follow God's will. James 4:7 says "Submit yourselves therefore to God. Resist the devil, and he will flee from you." First submit to God and then He will help you resist.
Don't threaten to tell his wife what has happened unless you intend to follow through. For the most part, it is better she not know (if she doesn't already). Don't let him make promises he can't keep. Don't let him use the past against you; if God has forgiven you, don't let him hold that sin over your head.
If his contact with you is primarily by phone, see about changing your number and having it unlisted. It may be a hassle, and it may cost a small fee, but it will be worth it to be rid of him. If he keeps calling you after you tell him to stop, going to court to get a restraining order may be necessary. It may be a public measure, but would at least put the law on your side.
Remain firm, and seek help from a local congregation, preacher, or counselor. Other Christians can help you overcome temptation if you let them. If you must meet him, taking someone along will help you resist his temptations.
Please note that I am not a qualified marriage/family counselor. My suggestions here are merely practical thoughts that you may choose to accept or reject, but you may need professional help to deal with this.