This recently happened to me. Still trying to move on from it all. They helped me out when I was going through a break up and I've stayed with them while they were going through an abusive relationship of their own even when no one else wanted to listen or help. For well over 4 years we were best friends and decided to be in a relationship. Things were fine, and fights seldom happened. Then she got a job offer to teach in another country for a year. I agreed thinking it wouldnt be so bad for a year. Overtime i heard from her less and less. Conversations became surface level but most of the time she'd say how much she missed her cats and family. If she missed me it was because she wanted something, not for me. I made plans to come visit in December because she made continuous comments about how she wished peopled would visit. When I told her the plan her only reaction was "I hear the trip going home is harder". I tried to stay positive but then she said she was going to stay for longer. Didnt want my opinion on it. When I asked why she said "Its not like I'm missing out on anything important". I tried to address concerns about things between us but she didnt want to hear about it. During this time I was question a lot of things. When did this start? Have I just not noticed this? Am I doing something wrong? Questions that didnt seem like that were going to be answered. So I ended it. The weeks leading up to it the whole time felt like she was asking for it to end but didnt want to say it. For 2 months I blamed myself thinking maybe I was wrong. I couldnt sleep, I had a difficult time processing that I even did that. Death honestly sounded better that enduring another day. But I also decided enough was enough. I started seeing a psychologist and theyve slowly helped me unravel it all and let me come to the realizations on my own and accepting what happened. Im sad that the relationship is gone but what feels worse is someone who I considered my best friend isnt the same person anymore.

Here is what I've learned in a quote

"It is possible to commit no mistakes and still lose. That is not a weakness; that is life." -Captain Jean-Luc Picard

So yeah, all in all the worst thing you can do to someone is to get to know them inside and out and then act like they are against you. Let them be consumed by their own doubts and fears. Never offer reassurance