Down deep I'm sure your mother knows that you can possess real feelings for boys and that teenagers can and do have real love lives. My guess is that it is fear that is causing her response. She may have suffered and made mistakes in her teenage love life and is trying to prevent you from making the same mistakes. These are just guesses, but many parents consciously or unconsciously respond to the appearance of love lives in their teenagers by clicking into fear and the memories of their own teenage romantic experiences.
Write your mom a letter, nicely explaining how her response to your feelings hurts you. If you truly would like to share all parts of your life with your mom (or at least most of them), including your love life, tell her so and tell her that you don't want to cut off the good relationship that you've had in the past. Let her know how important her understanding and unconditional love has always been to you. In essence, ask her for what you need in this area -- understanding, respect, and maybe even some good advice now and then. Try to understand that dealing with our children as sexual beings is a difficult thing for parents to do. All you can do is to try to put yourself in your mom's shoes, tell her what you need from her and why, and trust that the love between you will help things get better.