I was talking in a conversation and I thought for a split-second " well, its probably wrong" without thinking about mortal and venial distinctions or thinking so-be-it if its mortal, just generic bad, and then I said the thing. It ended up not being grave matter. Is such a mortal sin?
Please get spiritual direction from your pastor. It sounds as if you are having trouble discerning normal things from sins. This could be an indicator of scrupulosity.
I second this.
It would be better to get advice from a trusted priest or counselor, as well meaning advice from the internet or other places may not fully apply to your situation.