I'm currently 23 (I think...?) but my CPTSD is related mostly to a kidnapping that lasted about a year, living with my psychopathic father, neglect, and other technically-unrelated events. I did just get out of a relationship that was abusive, but I'm honestly not sure it would be considered a "normal" situation that I could find others who suffered with it. Unless your ex had you make up a world for them and prove over and over it was real, with death and suicide threats. *shrug* I honestly hope my ex gets better.
I AM the same age as you, sort of (at least close enough) and I understand what it's like to grow up with PTSD. I was diagnosed with PTSD at age 13. I know of at least a couple of other people who are the same age. If that's helpful at all :)
As for relaxing, that will take some practice. Start with mindful meditation, maybe -- have hard candies around, and use them to ground yourself. Try eating one when stressed, and focus with words in your mind on the flavors (try picking them out!), texture, temperature, structure, shape, all the hard or sharp parts, the urge to chew if you have one, etc. Do it a few times when you aren't as stressed. It will help you ground. I have occasionally used cold showers -- science has shown that your brain will react to cold water in such a way that you will get a "reset" of emotions (I can try to explain that if you're curious) which can help you relax. Try reading a book, or try watching a movie you've already seen before.
After you get yourself calm enough, reflect on how you reacted and what was triggering it, if/when you're ready. This can help you react less the next times, eventually.
Your psychiatrist may help you with medications.
Keep in mind that while you were going through that massive stress, you learned that danger was everywhere that had been safe before. Your reactions are normal and so is your stress. Luckily the cause of your stress won't be around while you're trying to relax. Say things like, "I'm safe, right here and right now," to yourself. It can help.
I hope that helps :)