My best friend abandoned me after 3 years then got together with someone she had her eye on a week later

Hi, I'm kinda new here and I'm going through some relationship stuff right now. About a month ago, my other half of 3+ years dumped me out of the blue over the phone, saying she hadn't been interested in me at all in the last year but had her eye on someone else, who she got together within a week after dumping me.

I was coping well with just the breakup, but knowing these extra details has completely destroyed me. This person who I had been through so much with and trusted and loved so much had been playing this facade for the last year and just abandoned me when they couldn't be bothered anymore whilst they already had an escape route planned with someone else. It's proving really hard not to miss this person who left me for someone else. She had a year or so to lose her feelings for me but mine still linger and this is all too sudden for me. She was my best friend and I just don't know what to do anymore. She still wants to be friends but I can't even think of her and Mr Perfect anymore without crying or having dark thoughts. I have no joy in anything anymore, I just don't enjoy being alive right now. Dumping me out of nowhere over the phone was so sudden, and I can't believe her when she says she cares about how I'm feeling knowing she replaced me in a week. I feel like she'll just care about me until I'm not suicidal anymore then she'll just forget me and move on cos I'm not her problem anymore.

I tried talking to some friends but they all just said the same stuff, "It'll get better over time", or "there will be someone else". I feel like they aren't treating me seriously and I'm scared to seek help because I'm worried they'll just tell me to suck it up and move on. And I definitely can't talk to my parents about all this. I tried talking to a therapist here, but after an hour we realised it wasn't going anywhere so he gave up.

What can I do? Does anyone have any advice on getting through this? I'm really struggling to see a light at the end of the tunnel.