I feel like if I go above my current intake, I will gain a lot of weight because I have done in the past. Have I completely ruined my metabolism by eating this little? How can I fix it?
I would be really grateful for any replies



I will also do some more research on what is a healthy goal for me, as my view is pretty skewed at the moment.

You should be speaking with a therapist/registered dietician who specializes in eating disorders. Have you ruined your metabolism? No (ruin implies permanent damage) but there has almost certainly been some metabolic damage (although water weight retention is the most likely culprit of raising calories beyond 500 - your body's system is really out of whack from long term deprivation).