"Get over it"
Angrily saying "get over it" has never helped someone move past pain. If anything, it makes them feel attacked by someone they felt safe enough with to share.
Be very careful before uttering those words.
Words echo and can't be erased even with an apology. If you ever feel frustrated or tired of hearing someone complain about something you think they should be "over," you can suggest they see a counselor or ask them if it's ok to talk about more positive things with you because although you feel for them, it's upsetting to you.
You can make them aware with tact and care that the emotional burden is too much for you if it is.