I don't regret any, and I've sold quite a few rifles and handguns.
If I decide to sell it, it means I'm not attached to it. If I had to fund something in an emergency, there are many shooter grade guns that would go before the ones I "can't live without".
Overall, I have come to become less attached to material objects. You can't take any of it with you, so shoot them and enjoy. Once in a while I go through a "selling phase" to add more of what I enjoy. I was an avid rifle shooter for 10 years, over the past year I sold most of my 100+ piece rifle collection so I could pay off debt, and in the process added more S&W and Ruger handguns to my collection.
I don't live my life in the rear view mirror, you can only look forward, not back. Don't dwell on selling something that you were attached to, just think about what you may get in the future that you like as much if not more.