When you are having one discussion with someone, but also have another unresolved issue with them, you are bound to mix one issue with the other. In other words, your comments about the new issue are actually about the old issue.
This mix up will not help you get the best discussion and the best outcome on the new issue. What is the solution? Deal with the older issue first before embarking on the new issue. If this is not possible, then at least acknowledge to each other the impact of the old issue on the new discussion.
The above is not easy to do because of emotions involved. People with strong emotional intelligence are able to master their emotion and deal with this challenge objectively. However, even for these type of people, it will not be easy to achieve this without a heightened sense of self-awareness around this problem.