I faked a melting point on a lab report and I feel so guilty.
Why do you feel guilty about this?
It's on my conscience and it's fucking up my life.
Feeling guilt over making an intentional misrepresentation on a lab report is perfectly normal behavior. You realize that lying is wrong. If the lab report is significant and has the possibility of affecting others, you should go and make a correction so that your conscience is clear.
The rest of the data is real and this one misrepresentation won't be harmful. I just don't want to admit it to anyone.
You know what you did was wrong and ultimately the negative effects are minimal. Chalk it up as a lesson learned and don't do it again.