It sounds to me that your situation is very good (enviable in fact) and that you logically know that. And so the problem is that you are generalizing an issue that occurred in a specific circumstance with a specific person to situations that have the potential to be characteristically similar, but is in fact very different qualitatively speaking. I give you a lot of kudos for recognizing this.
It's a natural thing to do. A more simplistic idea, but a good analogy is like when a person gets the stomach a flu caused by a virus, but our brain blames the last thing we ate, and feels like that food should be avoided in the future. So your brain is looking at situation and getting all freaked out blaming the wrong thing.
Really, it's kind of like a glitch in your programming.
As silly as it sometimes is, I find affirmations to be helpful. And in your case, they need to come from your partners. For example, when they are getting ready to spend time alone, if they would take just a few minutes before embarking on their alone time to tell you how much they love and value you, perhaps at the same time encouraging to pamper yourself while you have time to yourself. Kind of like a ritual. I have a feeling that if you got that little bit of reassurance right before, it might began to over-ride the emotion. You'll have been showered with good feelings and begin to associate their alone time with an expression of their love.