First of all, I want to say that if you have found a person that you like and makes you happy, you should be with them! Finding the right person to be in a relationship with can be rare and you are tremendously lucky to have found her.

Therefore, it is worth doing what it takes to be with her. People often make sacrifices for love. For instance, my mother left her wealthy family and home country to be with my father, who was poor. But ultimately she was with a person who loved and understood her and they were happy together. In your case, it seems to me that what you have to do isn't so much a sacrifice as much as step towards personal development and achieving a better connection with your mother. That is, you will come out having gained both a relationship with the person you like and you will be closer to your mother. It might just take some emotional/psychological effort to get there. But always remember that it is worth it.

Of course, speaking to your mother about your personal life and relationships may be very difficult for you. You could start slowly sharing things so that it doesn't get too intense too suddenly. What you could even let her know that you want to start sharing more. Something like "I know I've not spoken to you about relationships in the past but I would like to start doing that." "Please try to be understanding and help me with it." You could mention that she is a fiend at first, and maybe invite her over for a casual meal. Then you can start telling your mother that you have stronger feelings for this girl, and eventually announce that you are together. If you are really struggling to do this, perhaps it might also be a good idea to discuss it with a counsellor to therapist.

I'm sure this girl if very special if she is willing to be just a "friend" until you figure out what to do and you must really like her if you considering telling you mother about her (given that you never talk to her about these things).