I would say it is appropriate to say "I couldn't get it to this day", but that might be confusing for others to understand. Other expressions that get the same idea across are: "It scarred me for life": This is a common expression used to say that something you saw or experienced really imprinted you permanently, much like a scar. "It's like it happened yesterday": This is a common expressed used to say that something you saw or experienced a long time ago is still a vivid memory, as vivid as your memories from yesterday are. The reason I like these expressions better than "I couldn't forget it until that day" is because I feel they capture your meaning a little more (the meaning you wrote in brackets). "I couldn't forget it until that day" implies that there finally came a day ("that day") when you were able to forget it. If I understand you correctly, you mean that you just can't forget this bad thing that you saw. That is why I would use one of the two common expressions above. Hope that helps! Message me if you have a question.

I couldn't forget it until that day.