[Serra Britt]

My therapist gave me a list of what he calls "thinking distortions". What yours seems to fall under is "Disqualifying the Positive" and "Catastrophizing".

Basically, it appears that even though you have a good relationship and plenty of experiences to back that up, you still think about something else that might happen and then think of the worst possible outcome.

As for how to overcome it, maybe something to remind you of the truths of the relationship? That's something I do when I start thinking negative thoughts, is to back up a bit and remind myself of what the truth is.

Hope my advice helps a little.