Overbearing mum?

i am a young mum, married and good at my job. i have a really close relationship with my own mum, who I see several times a week. the issue id now that when I go some where with her, she takes over parenting my sons. i let her do this with my eldest who is 18 months, as I slip into the child role when I am around her, and I do not feel like a mum, just her daughter. My eldest son now only wants her when we are together, and goes to her for all his needs. he does not let her outta his sight in public places, and has separation anxiety from HER, and me when we are not with her. this makes me feel awful, as he is my 1st baby and has had nothing but love and somtimes too much care from me. i feel like he does not want me, only my mum, and I do not want this to happen with my second son. any advice on how to rectify this situation and be "mum" when around my own mum? i do not want to ruin the grandparent relationship, but I need to be the parent? advice please!!!