Just a short word of encouragement: I don't know if this works for your partner as well, but I never think of my other partner when I am intimate with one of them. My mind is fixed on the present one, because otherwise I wouldn't be "in the mood" to be intimate in that situation. Has he told you that he is thinking about the other or are you assuming this? Maybe you are wrong and worrying in vain.
And yes, it sometimes is hard to stop at the right moment and not talk too much. I am glad that I do not slip often but it happens. I am sorry that you are hurting this much. Whine as much as you need to, it relieves stress. All the best for you.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.