27f wife doesn’t like to get on top of 29m

my 29/M wife 27/F 6 years long relationship, does not prefer to be on top, she knew an ex gf and I guess heard stories from her that intrude her mind when doing the deed. (understandably) I try to help these thoughts (by not talking about them or creating situations to provoke them) but that is the excuse when I ask her to get on top. Is there any advice to make her want to get on top of me and feel comfortable doing so?