Torn. I Need Advice Please.

I am in need to get the best advice I could get, because I am in the toughest situation of my life. Here's my situation... My nephew is like a son to me, I love him so much. I am 25 and he is 6. His father is now in prison, and his mother (my sister) isn't a fit mother because she is not right in the mind. She also lives out of state with our dad. My nephew and I have lived at my mom's house our whole lives. We have never ever moved. He loves living here with grandma and I. I have also been in a relationship for over 8 years, but my boyfriend has been in the Air Force for about a year now. We have a great relationship and a lot of fun together. He wants to get married very soon, and I'm happy because I've wanted to get married for a few years now. The thing is, since he is military now, he isn't local anymore. He is stationed in North Dakota and we're both from California. I feel so torn and sick to the stomach, because I know my nephew would be devastated if I moved away. My mom and I are pretty much the only people who have raised him and taken good care of him and love him like he deserves. I feel like if I moved, it would affect him deeply, because I am like a mom to him and I know he is attached to me. I am very attached to him also, I love him like a son, and I feel I would regret leaving him and possibly damaging his little mind. I'm scared to leave my mom too. Like I said, I have never moved before so it's hard. It might be different if my nephew's parents were fit people who took care of him and lived normal lives, but unfortunately it isn't that way and all my nephew has is my mom and I. I do feel if I stay to raise my nephew, my relationship will be lost. I really do want to marry the man I love so much, I don't want to think about losing him. But my nephew means so much to me too. I know I could never adopt my nephew because my sister is too selfish. I'm just scared to leave home for the first time, and I fear I will become very homesick and regret moving away from my nephew and mom. I know it would be hard on my mom, especially without my help. But then I feel I would regret not marrying my boyfriend of 8 years and starting our life. I wouldn't be torn if my boyfriend still lived in California, so that I could have my marriage with him AND my nephew and family. Please help!!! I am so torn. Life and it's choices... :(