Is living together before marriage a sin?

It has become such a norm that couples live together mainly for convenience prior to getting married. I am no different. I have lived with my boyfriend for almost 2 years now. We have talked about marriage, but due to us working in the same company and he being my director, we have put marriage on hold. I want to be married and he tells me that it will happen and that everything will work out. I feel guilty that I am enjoying all the conveniences of living together when it is a sin. It eats at me everyday and though most days I am content, every few weeks or so, I break down and it makes me very impatient to be married. I don't know if this is right or wrong. Is it just me wanting the recognition as his wife and the security of a marriage or is it really my desire to not live in sin? Could it be both? If so, how can I tell that I'm not using the sin part of my fear to justify all my emotions? I love God very much and I want so much for him to be proud of me and I feel that living with my boyfriend is preventing that from happening. Then again, I feel like I'm too weak to walk out. We just bought a house, I am financially secure. I want to start graduate school soon and if I walk out, everything will be lost, won't it? I don't want to force him to marry me. I feel that it should be something he wants to do. I don't want him to resent me should anything go wrong. Where do I draw the line? I'm so confused. Can you help me, please?