If you and your girlfriend are living together and enjoying the benefits of marriage, such as sex, then you are living in sin. God is not pleased. Just because you are happy and your life is good it does not mean that God is blessing it in the sense that He accepts your sinful lifestyle. See what He says in Matthew 5:45.
We know from the Bible that there will be no such thing as marriage in heaven (Matthew 22:30), and therefore we know that marriage was instituted by God to be a thing on the earth. What makes a marriage valid? Well that depends on where you live and where you get married. You should follow the laws and rules of the government where you live as long as those laws or rules don't ask you to break God's laws and rules. In most countries, a marriage is valid once it is legally recognized. For this reason, there is no such thing as "spiritual marriage" where two people promise each other to be "married" but there is no legal recognition of that promise. (It would of course be different if the law said that it's a legal marriage if two people say to each other that they're married.)
So, I encourage you to repent from that sinful way you have chosen to follow, seek the counseling and advice of a good pastor and church, and follow God's Word.
It isn't illegal to have a government free spiritual marriage. So we've violated no human law.
The institution of civil marriage isn't governed by the Word of God. As believers we would have to go before the courts of the unbelievers should we divorce. We are commanded not to in I Corinthians 6:
"3 Do you not know that we are to judge angels? How much more, then, matters pertaining to this life! 4 So if you have such cases, why do you lay them before those who have no standing in the church? 5 I say this to your shame. Can it be that there is no one among you wise enough to settle a dispute between the brothers, 6 but brother goes to law against brother, and that before unbelievers? 7 To have lawsuits at all with one another is already a defeat for you. Why not rather suffer wrong? Why not rather be defrauded?" ~ Paul, I Corinthians 6:3-7 (ESV)
We will not be beholden to unbelieving courts.
Not only that, but the institution of civil marriage is being expanded to homosexuals. We refuse to be associated with that institution.
The Quakers traditionally had self-officiated, government free, marriages. The challenge shouldn't be for us to bow to the dictates of the state concerning the marriage... the challenge is for the state to begin honoring strictly Christian marriage contracts.
Frankly, our marriage is more biblical, historical, and superior to anything the state has to offer.
And yes... many recognize our marriage before God. The STATE isn't God. Render unto Caesar that which is Caesar's... and render unto GOD that which is God's. A Christian marriage belongs to the LORD ("...what GOD has joined together...).
If it is "sin"... are countless Quaker couples living in private marriages without state marriage licenses "in sin"???
The Quakers are right on this one. Marriage is a disaster because the STATE has essentially hijacked it... redefined it... and set ungodly standards concerning it. The STATE ruins nearly anything it touches. And if you look at your history... the state only took over marriage in a mad effort to keep whites and blacks from marrying. Prior to that... Common Law Marriage was universally recognized by both the church and state.
We are not owned by the state. We are private citizens. And our marriage is between ourselves... and our God.
Selah.
P.S.
Please note... in the following states... our marriage would be recognized and legally valid:
Alabama
Colorado
Kansas
Rhode Island
South Carolina
Iowa
Montana
Utah
Texas
So according to you... God has different rules for those who do not live in the above listed states. I find that problematic. God is no respector of persons based on their state of residence. And God doesn't bend His will to accomodate corrupt human governments.