I'm just so tired of it

I don't know what's wrong with me. I mean I guess it's because I still love her and feel like we missed out on being together just because I wasn't emotionally stable enougg to realise I loved her but still I just don't understand how I can still be stuck on her, thinking about her daily from the moment I wake up, even though she left for another guy back in May. I just feel so stupid to still be struggling when she's living her life as if nothing was lost.

We were best friends before we started seeing each other, I was there for her when she left her previous boyfriend because he was a serial cheater, we stayed friends with benefits for a year, talking about making it official towards the end, and two weeks after I brought it up again and we agreed to try she went on a date with a random giy she met on Bumble, with whom she had been talking for three months.

I just don't get it, I've never been in so much pain over losing someone, I've never had a breakup leaving me so utterly devastated. I do my best to take care of myself but the truth is I've lost the energy to do anything. I had started working out again, engaging with my hobbies, keeping busy but recently I struggle to even cook for myself, I just want her back and it's killing me because I know she just doesn't care.

I just don't get it.