Just a few thoughts that might help some folks out.
Be patient with her, you don't want her to do this before she is ready.
Try and introduce cuckolding porn into your forplay sessions, after you watch it together and u tell her what makes you hot it may help a little.
Instead of trying to get her to just jump right into the sack with another man, go out with her and have her mingle and flirt while you watch from a distance. This way it is like baby stepping into the idea. If she likes the attention she may open up more.
If you have a small dick, then ask her about what she thinks a bigger one would feel like. Could get her thinking dirty about others more.
Introduce dildos and other toys into your sex life, you can roleplay and watch while she fucks herself with "another cock" than yours. You could even act out a "clean-up" from the imaginary creampie. This will help her to think of other cocks and see what it is like to have you submit and worship her.
Last, have her take control of you for a weekend and let her do whatever she wants with you. You can be like her slave and her your Mistress. You may be surprised at what things she would like for you to do. :shame:
Hope this helps.