fiance wants to help me pop my girl on girl cherry

Ok so I identify as bisexual as I am attracted to both men and women, but other than a few kisses, my only sexual experiences have been with men.. my fiance and I are very open about our sexuality with each other and he is very encouraging about me finally experiencing more with a woman.. but my problem is that since I have put on a lot of weight over the last few years I have ZERO body confidence and the idea of being with someone other than my fiance terrifies me.. so how did you all deal with first time anxiety and do you have any advice on how to get over it