I detest ANY kind of comment that implies we can all just decide how to feel and feel that way. Wtf, do these people think humans come with a dial that can be set to the emotion of our choice? Ordering someone to feel X instead of Y is just plain stupid, on top of being rude and insensitive, and doing that to someone who is dealing with trauma and/or whose brain chemistry is out of whack for reasons beyond their control is just plain cruel.
I don't think most people mean to be rude, insensitive, or cruel when they say things like that, but one's intentions and the effects of one's behavior on others cam be VERY different things.
Some people say things like "stop feeling sorry for yourself" or "cheer up" out of a genuine (albeit very misguided) desire to help. Others say those things because they're tired of hearing about someone else's misery and just want to make it stop.
There are also people who are deliberately inconsiderate or even cruel (and some of us unfortunately have more people like that in our lives than others). This group may do the most lasting damage, but the first two can also do a lot of damage without intending to.
I wish parents would do a better job of teaching kids how to manage their emotions, but it's a hell of a lot easier said than done, especially for those who have never learned how to manage their own emotions properly (like my parents). Too many of us never have a good model of a well-adjusted adult to follow or get our own needs met sufficiently while growing up, and we have to spend the rest of our lives figuring out these things work.
When I look back at the mistakes I've made in my life as a result of learning the wrong lessons while growing up, I can get very frustrated about all the things that went wrong because I didn't know how to handle them better. I'd be better off if I could remember to give myself credit for the lessons I've learned and the work I did to learn them, instead of resenting the fact that I had to do all that work. I suspect that's true of a lot of us.