This is my take on your situation.
If you do nothing it is apparent that you will either end up permanently in a psychiatric hospital or you will kill yourself. You are on fire on the inside and you currently have no way to let go/forgive yourself for what has happened. Yes, what you did was wrong but your current approach will kill you for sure.
What I would do if I were in your position would be first, see a lawyer and ask for advice. Let him/her know that you want to end what you've been doing and that you want psychological help and want to settle this matter regardless of the 'legal' price that you have to pay. I'm not sure if a lawyer has to report what you have done to authorities but I do know that a counselor or psychologist/psychiatrist does. A lawyer can let you know about that and each state has different laws in that regard.
Again - see a lawyer first, counseling second and open up and face the consequences. If you are without funds there are still avenues available to you for both of those things for indigent people.
Now what you have done is indeed serious but don't kill yourself over it. The "good" news in what you have written is that you recognize that what you have done is wrong and you wish to correct it. That's a plus. There are many out there in this world who would just continue the abuse and hide and bury it. Yes, you're going to face family shame and possibly even incarceration but you have to face what you have done. If you don't the consequences you are placing on yourself is worse than what will happen with family and courts.
I'm a guy who was sexually abused by family members as a child and they have never owned or asked for forgiveness for what they did. Yet ultimately I have had to face that abuse and forgive them. It's not easy, it's a journey, but the path has been one humongous healing for me and other can and have done it too. Forgiving my abusers is my responsibility not theirs. Look at this as an opportunity to heal yourself.
RegretStuff, I just saw your post and I haven't read all of the responses, but I'm going to make a suggestion that you consider getting some help for yourself before you do this, again - and, it is inevitable that you will, and I'll tell you why.
You stated that you had porn on your cell phone and this is an indication that you may have a very unhealthy understanding of sex, love, and sexuality. Only in the past 15 years has pornography become easily accessible for individuals to entertain this in their own homes, at work, or on a city bus. Hard-core pornographic imagery used to be available ONLY by traveling to unsavory places in the dead of night and parking around the back. Today, every manner of deviance is available at the click of a mouse or the tap of a finger on a cell phone at any time, and in any place. Pornography has become an "acceptable" reflection of human sexuality, and it is not even remotely an expression of healthy interaction. I'll hazard a guess that you've been viewing porn for many years? Porn is an industry, not a reflection of healthy consentual sex.
That you knew what you did to that child was "wrong" only goes further to suggest that you need to get involved in some serious counseling and address your core-issues before you do something "wrong" to more children. Regardless of how you may feel about your actions, you committed a sexua| crime against an innocent child and your words support the strong likelihood that you'll do this, again, because you are stating over, and over, that you aren't the type of person who would do this when you clearly are by your own admissions. If you engage in some strong counseling, you will NOT be judged or arrested, but you may find answers and healing from whatever gave you the impression that you are allowed to molest a child, even though you knew that it was "wrong." I am not being judgemental, here - simply factual.
If you choose to wallow in your "regret," it's not going to do you any service except to compel you to molest more children. This is also a fact and not a judgement. Get help for yourself and break your addictions before you end up in prison. And, I will simply say that child molestors do not fare well in prison environments.
I wish you the best of luck and sincerely hope that, if your post is true, you will be one in a million that does something before you harm another child. Here are some sites that may help you:
http://www.wral.com/news/local/story/138638/
http://www.goodtherapy.org/blog/child-molester-rehabilitation-therapy-0615126
EDIT ADD: The previous responses that you "did nothing wrong" are wholly, completely, and utterly erroneous. You performed oral sex on a child and that simple act will paint this child's view of herself, her self-worth, her sexuality, and her self-esteem for the rest of her life. Sex is not defined by penis + vagina. People experience sexua| gratification without vaginal intercourse ALL of the time. So, please, don't fool yourself into believing that what you did wasn't "sex." And, the fact that you are laying a veiled "blame" on this child because you simply resonded to her request for your sexua| attention is a huge "Red Flag." Get some help.