Turned upside down

Hi all
Im in a very dark place again. After a month or two where the intensities of my anxiety and depression eased slightly. Im in a terrible terrible way.
I was triggered by an ex girlfriend. I haven't spoken to her for 10 years. We went out for a few months and I broke it off as I was concerned about the age difference. She was 23 at the time and I was 32 - not huge difference I know, but I convinced myself it wouldn't last, even though I thought the absolute world of her. I thought about her a lot since but never been in touch. I tried to move on, I am currently married with a daughter - although the relationship isn't great and we have been discussing splitting.
As soon as saw she messaged me it was like a jolt through my body, she was only asking how I was doing, nothing more than that, but hearing from her has literally sent me reeling. We chatted a bit, she's got a long term partner and seems very happy. I've been left with such intense feeling of remorse and sadness, like nothing I've experienced before. I haven't slept for days. Perhaps this reaction is more to due with the difficulties at home - but its literally killing me. I feel like I just want to die to escape the pain of regret, and the anxiety it is causing me. Why I am so so ****ing sensitive to stuff like this - every time a I take step forward, bang something hits me a like a train and Im completely derailed.