You say you are Christian and so I will assume you are not simply saying you believe in God (which is what I think some people mean when they say this) but rather that you have accepted Christ as per the teachings of most mainstream Christian churches.
I am Christian and have stopped attending church because I am so ashamed of myself
No need to feel ashamed. Church's are filled with people who have sinned and will continue to sin. Find a church that is preaching God's message of salvation and all that goes with it. Seek forgiveness and it will be given. That is the central message of Christ.
That said, as a Christian, you know that your current living situation is not one in which God approves and it will keep you from having the relationship with Him that He desires. You need to decide whether the sexual relationship with your boyfriend is more important than a relationship with God. If it is, then keep on keeping on. Otherwise, you need to think seriously about talking to your boyfriend about why you feel it is important to get married. But before this conversation, you need to decide whether your faith is more important than your relationship. Because if it is and you feel strongly enough about this, you may need to start sleeping in another room until he puts a ring on your finger. ( I know a couple who had 2 kids together. They began attending church and both became Christians and were committed enough to their faith that they immediately began sleeping in different bedrooms until they got married six months later. Neither of them has ever regretted the decision to do so and claim it was the best thing to ever happen for their relationship to each other and to God.)
Sorry for being long winded. Whatever you decide, good luck to you.