I fornicated. I feel so horrible, especially the fact that it wasn't with my Christian boyfriend. It was with someone at my job. I don't know what to do. I knew what I was doing was wrong, but I couldn't control myself. Now I feel horrible because I haven't told anyone what I've done, especially my boyfriend of two years. I'm scared that I won't be forgiven by God. What can I do? Please help me.
You are not feeling guilt because you have not told anyone you sinned. You are feeling guilty because you sinned.
There is a false statement in what you wrote. You stated that you could not control yourself. That is not true. "No temptation has overtaken you except such as is common to man; but God is faithful, who will not allow you to be tempted beyond what you are able, but with the temptation will also make the way of escape, that you may be able to bear it" (I Corinthians 10:13). I suspect that you allowed the temptation to blossom into sin and it was only then that you realized that you needed to stop, but you were caught up in your desire to sin by that point.
While you state that you are scared that God won't forgive you, it seems you are more concerned that your boyfriend won't forgive you when he finds out what happened. But God is really the one you should be concerned about.
The proper question is whether you are going to change your attitude and behavior that allowed you to fall into this sexual sin. You state that you feel horrible, but feelings are changeable. Are you going to remain as you are, or are you going to change for the better. "For godly sorrow produces repentance leading to salvation, not to be regretted; but the sorrow of the world produces death. For observe this very thing, that you sorrowed in a godly manner: What diligence it produced in you, what clearing of yourselves, what indignation, what fear, what vehement desire, what zeal, what vindication! In all things you proved yourselves to be clear in this matter" (II Corinthians 7:10-11).
If your boyfriend asks if you have had sex, you will have to be honest with him. In this case, I believe you need to tell him because he is making assumptions about you that are no longer true. His decision about whether to marry you will be heavily impacted by what happened. Hopefully after he gets over his anger, your boyfriend will forgive you. Yet, the risk of him not forgiving you is better than his finding out after you are married that you had sex while dating him. In that case he will feel that he was falsely trapped in his marriage because you withheld information.