My Partner of 4 years broke up with me.

I am really struggling.

I thought my (23) ex partner (27) would've been my life partner - we had such a great relationship, until we didn't.

Me and my ex partner were in a bit of a complicated relationship. We lived in the same town for the first year, moved to a new city and lived together for 6 months, then she decided to move across the world to further her studies. We decided to have a long distance relationship as I was getting my life sorted to go and join her. As I was on the precipice of being able to join her - she dumped me, blindsiding me completely.

We had a long distance relationship for 2 and a half years (that is where the complication comes in) and in that time I made 3 visits to see her which all went really well (that I thought) and I know that long distance is hard - really hard. We both went through it.

The reason she broke things off is because she cheated on me by having a casual fling that turned into something more with a 21 year old (shes 27). She ruined a seemingly rock solid relationship over a fling with a 21yo that she caught feelings for. That seems like such a dumb fucking thing to do.

Like, I just can't fathom it. That we were so close - I know this sounds cliche but I love her and I know that she still loves me (her words). We had such a solid relationship, our relationship was her longest, on my second time visiting her she wrote me a song expressing her love towards me (something she'd never done to a previous partner) which was 11 months before she broke up with me. Both of our families get along really well, I get along extremely well with her family, all for it to end on her deciding to further a relationship with a 21 year old who was a casual fling turned relationship? Like how can someone possibly think that is a good idea? especially at her age. I just truly cannot fathom what has happened.

For reference, the reason I am having such a visceral reaction is because she was the first girlfriend I've had - first for everything.

I just needed this to get this off my chest - out into the world. I suppose I am also looking for advice on moving forward and overcoming my feelings I have about the situation.

Sorry for the wall of text, hope it makes sense.