I have used your service before and I cannot thank you enough for your efforts. What I have to ask is difficult for me. I committed a major sin and what Allah hides we should not talk about. I acknowledge my mistakes and acknowledge my weakness to Allah and ask for His forgiveness for my continuing to sin and I always hide myself and hope that Allah will not let my secret be known. I try to keep my Salat, fasting and reading the Quran and I try to help others and give charity. My questions are:
Can I hope that Allah would accept my Salat and good deeds despite the fact that I am committing a serious sin, or are all my good deeds not accepted by Allah? Is there any particular sin (except for suicide) that a Muslim becomes a Kafir by committing it?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

A Muslim's duty is to follow all the rulings imposed on him from Allah and avoid all prohibitions set by Allah. He should try his best to do good and avoid evil. This is the meaning of Taqwa which is demanded by Allah in many verses of the Quran. Whoever ignores the Rulings of Allah and transgresses His Prohibitions he is considered as disobedient person. Such person should repent as soon as possible. If he dies and has not repented from committing sins or abandoning an obligation, i.e., dies while he is disobedient to Allah, such person is under the Will of Allah. If Allah wants to forgive him He can, otherwise he deserves punishment to the extent of his disobedience. This ruling applies to all sins and violations except Shirk; since Allah never forgives the one who fails to repent from Shirk before his death. Know also that evil and sins are of two kinds: Major sins and minor sins. The major sins are unforgivable unless one makes repentance towards Allah, while the minor sins may be wiped out if one avoids major sins and practices good deeds.

Allah Says (what means): {If you avoid the great sins which you are forbidden to do, We shall remit from you your (small) sins, and admit you to a Noble Entrance (i.e. Paradise).} [Quran 4:31]

Abu Hurayrah, may Allah be pleased with him, narrated that the Prophet once sat on the pulpit and said: “By Him in Whose Hands my soul is, everyone who performs five daily prayers, fasts the month of Ramadan, pays the Zakat, avoids the seven major sins, except that eight doors of Paradise get opened for him at the Day of Judgement then the Prophet recited the above verse.” [an-Nasaa’i, Ibn Maajah and Al-Haakim who graded it as a Sound Hadeeth).

The Prophet also said about the seven major sins: “Avoid the seven great destructive sins.” They (the people) asked, “O Allah's Apostle! What are they?" He said, "To join partners in worship with Allah; to practice sorcery; to kill the life which Allah has forbidden except for a just cause (according to Islamic law); to eat up usury (Riba), to eat up the property of an orphan; to give one's back to the enemy and fleeing from the battlefield at the time of fighting and to accuse chaste women who never even think of anything touching chastity and are good believers.”

Finally, we urge our brother to sincerely repent, give up his sin even though it is a minor one. Persistence in minor sins make it a major sin. We warn you, too, that performing prayers, paying Zakat, and fasting (assuming that they have been accepted) does not mean that Allah will not call one to account. Despite good deeds, if one commits more sins than his good deeds, he would be punished for his sins as it is mentioned in the Hadeeth that the Prophet said: “Whoever has wronged his brother, should ask for his pardon (before his death), as (in the Hereafter) there will be neither a Dinar nor a Dirham. If he has done some good deeds, a portion equal to his wrong doings will be subtracted from them; but if he has no good deeds, he will be burdened with the evil deeds of the one he had wronged in the same proportion.” [Al-Bukhari]

Allah knows best.