I need to say something to be truly happy and not explode like a rocket

I am currently 15 years old. I now live with my father, and moved from my home with my mother's family almost a year now. My mother stopped acting like my mother years ago and has done many horrible things that mothers would not do, such as believe their boyfriend over her 3 kids when he touched them. During this time, my entire family revealed their true colors. Then, my dad who I had not seen for years comes back into my life to help me with everything, and ever since has been fantastic and has been always there for me. Even before my dad came back into the picture though, there was someone who was my field director when I volunteered, who I ended up venting to about everything going wrong in my life, who helped me through everything. This person truly acted like my mom when my mother did not. Throughout my life I have lived with my mom or my grandparents, and last year I moved dozens of times between aunts and my grandparents trying to avoid my mother who then lived with my grandparents (6 people in a 2 bedroom house.) Over this time, things were extremely rough to say the least. This person I am referencing was ALWAYS there for me, and at this time, I had no one like that, and hadn't for my entire life. Someone who actually cared and was willing to help me, because they saw me for who I truly am. At one point when things got really bad, she and her husband offered to adopt me, and would fight to the end with my mother who was acting much less like an adult. The only reason I truly declined the offer was because my mother being unpredictable and has been known to do crazy things, like call the cops on campaign members, claiming they had "brainwashed me." I did not want this person who meant and means the most in my life to be hurt. So, after the court trial, I had and have a protection order against my mother's boyfriend. I for the entire year almost moved in with my dad. This last Summer, I actually did it. And this person who has and had helped me all along helped with that in tremendous ways. This person is still and always will be in my life and I love that. But, this year so far, I have been rather depressed, because I have found it is different to live with a dad than a mom, and having a mom 24/7 is all I had ever known. I hate feeling this way, because my dad is fantastic, he has helped me through and is always there for me, but my heart is not happy. This person I have been referencing through this entire piece has for over a year now been like a mother to me, and I kick myself for not allowing her and her husband to fight for me back then. Now though, my dad has the right to say where I live, and my mother has no choice in the matter. And I have been wanting to ask this person if I could now move in with them, but am scared to hurt my dad and maybe hurt their feelings for them helping me get here and wanting a change. But being with this person, around their family, makes me happy and makes me feel at home, that feeling I have only lingered to feel. Tonight I did a big step by letting them know they are like a mom to me, but I always feel any sort of thank you from me is mediocre. I need to let them know how I feel and what I want to do, but I am shy, scared and cannot compose myself to tell them. I need to though because I feel like over a year now I have been bottling myself up and am over filled and am going to explode very very soon. Please help me figure this out, I need to feel happy, and I know how I will be happy, I just need help telling this person, who I am sure would be very open minded and has accepted me into her home any time. Thank you!