From what I understand your problems are threefold:
- Your mother doesn't see you as adult enough to make your own decisions.
- You and your boyfriend can't seem to live in the same country.
- You are afraid of losing him if you are physically apart.
Problem #1 is the hardest. You will never feel like an adult unless you can get your mother to back off and let you lead your own life.
You can either do what she wants and be miserable or do what you want and risk losing her. It's a very tough situation -- one that I'm working on myself!
Don't Do Things Behind Her Back
I can only say that you should be honest with her and not do things behind her back.
You need to figure out a way to get your mother to accept your choices.
Don't Worry About The Distance For Now
Problem #2 and #3 are related. I think that if you and your boyfriend really love each other that the distance will be okay in the short term.
As long as you have a long term plan to be together, you can sustain your relationship right now. My wife and I had a two year separation when we were dating and I think it actually helped us love each other more.
Talk To Your Supportive Relatives
Ultimately you are your own person. I suggest talking with your father or other relatives who DO support this relationship and try to figure out how to convince your mother that you are making the right choices and that you should be allowed to make your own decisions.