How To Discuss A Matter Without Hurting The Feelings Of Spouse?

I v e been a relationship for 12 years out of those 12 years I dedicated myself to this one person well we broke up about 3 years Was one of the hardest break up I ever had endured I gave myself 3 year to be alone to gather my thoughts to find myself again last year I found love again I care about this person deeply my question is how do I tell my ex I m in a new relationship without hurting her feelings because my new girlfriend is asking lots of questions about my old girlfriend like for instance why she keeps calling and I tell her to trust me because we re just friends so again my question is how do I tell my ex that I have a current girlfriend without hurting her feelings because she still means a lot to me?