I spent a lot of time killing the NCR guys for supplies. I am starting to regret it now as they are always attacking me when I am trying to do other things. Is there a way to make them friendly with me again?
If you wear the NCR gear they should ignore you, but obviously you may not want to be wearing this all the time to avoid being attacked. As with other factions, it is possible to do some quests for the NCR and in exchange they will agree to forget about all the murdering you have done in the past and be friends with you again. This is likely the best way to do it.