You don't need other people's approval for your feelings. They do not need to understand your feelings, in order to believe that they are real and important to you and to respond supportively, accordingly. Those seem to be the important themes here.
Don't seek others' approval of your own feelings, they're yours. And if your new BF can't understand that sometimes you will feel things that are very real and important but that he wouldn't feel in the same situation, or cannot imagine feeling as he lacks adequate life experience, imagination or empathy, then he's quite a limited person and lacks awareness of his own limitations. The important thing is how he reacts. Does he listen and support you? Or does he dismiss feelings that he doesn't understand?
On the one hand this habit of people only accepting feelings and reactions in others that they understand themselves is quite common. On the other it demonstrates low emotional intelligence, narrow-mindedness and rigidity. An unwillingness to learn and grow. Not good characteristics in a partner.
It also raises the important question 'who made him the judge of what is important in your relationship?'. Why is that ultimate arbiter not you? Or something shared, negotiated and made together, as it is between mature adults?
And do you want to live with that power imbalance forever? Placing power over your feelings in the hands of someone with low emotional intelligence, insight, empathy or compassion is not a good idea.
It's all about how he responds.
Obviously new BF has feelings too and seeing his new GF getting upset about something her ex has done, as if the ex is still the person she responds to most, won't be easy. I'm sure you'll have explained it's about the pain he caused you, not any residual desire to get back together. That pain and damage is your own experience and an important one, quite separate from the relationship that preceded it.