First of all, you didn’t stop him in the middle of sex. That’s literally impossible. The only possible time to stop sex is at the end. Because as soon as you stop it, that is the end. Anything after is rape.
Now that that’s out of the way, let’s talk about what you think of as the “middle” and why cuz I think that’s gonna be the root of why you feel bad. Now I’m gonna have to make some assumptions because I obviously don’t know for sure why you’re thinking the way you do, but if you’re like most people, you thought you were stopped “in the middle” because your partner didn’t cum.
Which is totally normal. In our culture, we’ve all been trained to view sex as a means to an end. A way of getting from point A (not having an orgasm) to point B (having an orgasm). It starts when a penis enters a vagina and ends when a cis guy cums.
But I’m gonna tell you a secret that will make your life so so much better. That’s bullshit. Sex is not a means to an end. Sex is an end. Sex is fun. You don’t have to have an orgasm. Your partner doesn’t have to have an orgasm. It starts when both/all parties say it starts and ends when one or more people say it ends.
And this is the hardest thing to unlearn. It’s something I still struggle with all the time. But you should never ever ever feel bad for stopping sex for any reason. Sex is supposed to be fun and defining it by an orgasm takes the focus away from the sex itself. Orgasms are nice but they’re not necessary. Not for you. Not for your partner.
In my own experience, have I stopped a partner before they’ve cum? You bet I have and I’m likely to again. For all sorts of reasons: I get tired, I feel sick, I get a back cramp, it starts to hurt, I plain don’t want to anymore, And I used to feel bad because like you, like so many people, I thought I was stopping in the middle, I thought I was negating the point of sex. But I wasn’t. Because the point of sex isn’t an orgasm.
TL/DR: Sex is not defined by someone’s orgasm. The end is not when a cis guy cums. Sex is an end in and of itself. The end is when one party says it ends. Do not feel bad about ending sex.