Wow, we (f/m) are so turned on watching you ride that cock! Went to a local sex shop looking for this magnificent blue dong and could not find it. However, I believe we found it online. Can't wait to try it out! https://www.amazon.com/gp/product/B00NHCFWNS
It’s a very common male fantasy, so there’s nothing unusual about your desire. But anal sex is NOT easy. Many many gay men don’t even like it.
Serious question: Have you been fucked with a dildo before? If not, consider buying one and getting fucked with it (use lots of lube) in the same way you might fuck her. I often suggest that gay men who are tops should get fucked so they understand the sensations—pleasant and not-so-pleasant— that they’re putting their sex partner through when they shove a cock up their ass and pound away. If you’re not willing to try this yourself, or you try and you don’t enjoy it, you really have no right to ask the same of your girlfriend.
That said, your girlfriend has tried and has told you she doesn’t like it and doesn’t want to do it. Might she learn to do it and enjoy it if you kept at it, started small, and worked your way up very patiently? Yes, absolutely. But that’s not really your call. She’s already stated her preference here. It sounds like you want her to honor your request, but you’re not willing to honor hers.
In the meantime, go get a dildo to use on yourself. I wouldn’t go bigger than this one: https://www.amazon.com/dp/B00NHCFWNS/ unless you yourself have a huge cock.
And you’ll need some lube: https://www.amazon.com/ID-Natural-Water-Based-Personal-Lubricant/dp/B00B1M3BSO