Please help, Any suggestions?

I’m not sure what to do, I am 15 years old and seem to be stuck in a rough patch. I am usually a very naturally energetic and cheerful person, and I hadn’t been “properly” sad in a long time. Until nine weeks ago I was on a bus and a man was sitting behind me talking very loudly to what seemed like a lawyer about his incidences with his partner, explaining how he had basically bashed her. It triggered some past memories for me and I haven’t been the same since that bus ride. I am constantly sad, if not sad I am angry. The smallest things set me off. For example during school the other day my teacher simply asked me to shut my laptop lid while he was talking, and I had to leave the class because I started to cry. That same night when I got home I accidentally stepped on a snail, once I realised I did I completely lost it and cried all night. I’m not sure how to explain it but it feels like a constant rotten feeling in the bottom of my stomach that won’t lift. In reality I know it will go away eventually, but for the most part it feels like it will never go away. This is quite possibly the most boring thing on here to read but i had to get it off my chest. Does anybody feel, or have felt like this that can relate? Or does anyone have any suggestions on what to do?