I want to know how to convince my girlfriend

Buy her a big dildo. Get her to cum with it. Fuck her and immediately lick the cum from her pussy and make her cum. Use the big dildo after you cum inside of her, after she cums feed her the remains, kiss her, tell her how good it tastes and then go down on her. Talk dirty about cleaning up after her 'boyfriend'. Tell her you sometimes fantasize about her and if she'd like her lover, another man, to cum inside her. Let her know if it ever happened you'd love to taste her. Role play a fantasy where she comes home from a girl's night and feeds you her lover's cum.