I need help

I have been best friends with someone for 13 years her family is my family. Something about coming out to people who have known me longer is so much harder then people I just met. My girlfriend and I have been dating for over a year and we HAVNT told her. We have always been a group of three and I feel so bad keeping her out and not telling her. She is in a wheelchair and when we go out my girlfriend and i take care of her. I feel like if I tell her know she will feel even more left out and hate us for not telling her. All our moms are friends and both our moms know about us dating and I feel like I force them to lie to keep our relationship hidden. I get sick to my stomach every time I think about it. My girlfriend would like to tell but she’s also very sweet and saying the kindest things which make me love her even more. But I want to be able to tell my best friend cause I used to tell her everything and now I don’t even want to see her cause I feel so awful. I just need some advice on what to do. If anyone can help that would be really helpful.