Don't take it too personally, the nature of the internet makes keeping contact with others in the long term difficult, as people can quietly slip away due to issues in their real life demanding their attention, and leave you with no real closure. Even if you hit it off with someone online, a lot of connections are very ephemeral and transient, especially if they have other things going on.
I knew someone online for 5 years and out of nowhere one day they just stopped talking and abandoned all their accounts. Knew their legal name and search them up occasionally to see if their name pops up in an obituary notice, but it never does. To this day I am unsure why they chose to cut contact with everyone online after so many years, but unfortunately this kind of thing does happen, even if you believe you are close friends and your bond is strong.
Did you have contact with them on any other social media or text them? Did they disappear completely on everyone or was it specific to your relationship? It's understandable why you find it so upsetting, with the promises of making a life together, but sadly I think some people (especially an individual in the throes of suicidality who desperately needs comfort) say things in the heat of the moment and don't realize there's a person on the other side of the screen who has an emotional investment in the relationship. It's easier to leave behind a digital relationship than one cultivated in the flesh.
I hope you can make some new friends soon, and hopefully if they needed to take a break from things they'd communicate with you. I know being left hanging really and truly blows.
