You are a smart young lady to be thinking this through and to ask for help. That says alot about you. And, of course, you know we always have advice.
I would wait until you find just the right place, in the right part of town, that you can afford on your own and feel safe in. You have made the decison to move out on your own. That is the first step. Now, while you are looking and waiting for just the right place, you can save your money. Have some extra on hand so if something happens, you don't have to break a lease that you are legally bound to pay, and go back home to live with mother. Breaking a lease is one of the first ways young people begin ruining their credit. Also, I would find a place to live that I could pay for myself and not have to depend on someone else to pay part of the rent. You never know what that other person will do. They could move out or cause damages, and stick you with the financial end of things. About living with your boyfriend....unmarried couples should not mingle funds. Someone always gets stuck paying more than their share because the car breaks down or some other story. You get mad at each other, break up, then one of you is left holding the bag. In short, if you can't make it on your own, stay at mama's house until you can. Save and plan towards it. You might want to get a part-time job to help build up your bank account. Good luck to you!