First of all, I would like to say that I find this website an excellent way to learn about the Lord and to be educated on the Bible, and I find myself becoming a regular user with regard to asking questions in full assurance that I will receive the answer I seek.
My question is that each night, I pray to the Lord that He forgive me for my sins and that He brings me happiness. I speak this out loud while laying in bed each night. I understand that nobody can possibly be 100% "sin-free," but no matter how much I ask for forgiveness, I still feel as if something is missing. As if I have forgotten something, or as if I have failed to be 100% honest even when I am sure that I have. I sometimes have bad luck in my life -- usually right after good luck. It can sometimes feel like it was meant to happen, like a punishment of some sort, but I just do not know why I feel the way I do. Is it possible that I have forgotten to confess a sin, or that I have sinned unconsciously? And if so, what can I say or do to ensure forgiveness and to put my own mind at rest at the same time?
There are two problems, one is that you are deciding whether you are forgiven or not based on your personal feelings. Feelings are deceptive. "He who trusts in his own heart is a fool, but whoever walks wisely will be delivered" (Proverbs 28:26). Rather we are told to trust what God has promised. "Trust in the LORD with all your heart, and lean not on your own understanding; in all your ways acknowledge Him, and He shall direct your paths" (Proverbs 3:5-6).
God said that when we confess our sins, He will forgive. "If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness" (I John 1:9). If you have talked to God about all that you know you've done wrong, then you've done as God has asked and you can trust that He will keep His promise to forgive you. If you've missed something, then eventually you will learn about it, and when you do you can talk to God about that problem as well. I don't know of any passage where God expects the impossible of men, such as confessing sins they are not aware of committing or didn't know were sinful. This doesn't excuse any sin, but God does understand us. We are to learn God's Word and as we learn, we realize that we had made mistakes. The answer is to tell God about it and make changes in our lives to not repeat those mistakes.
The second problem is assuming that bad things happen because we did something wrong. Sometimes that is true, but it isn't always the case. See Why Do Bad Things Happen? Since we can't know what is going on behind the scene, it is foolish to make assumptions about things we don't know. If you have an accident because you were speeding, then the smart thing is to stop speeding. If you get a disease because you were involved in sexual sin, then the smart thing to do is stop the sexual sins. But if something just happens for no apparent reason, then accept that sometimes such things do happen and go on with life.