It’s been tough…

I’m 28(m) my ex was 23(f). We’ve been together since January broke up in may. It’s been really hard. I mean yeah it’s only been a 5 month or 6 month relationship. But I don’t know why it’s so hard to move on from her… it’s been like 5 months already since we’ve broken up and yet she’s just always still on my mind. I’m just having such a hard time moving on and letting go is this even normal? Like this isn’t my first relationship yet I’m grieving so hard over her I just want to be happy again…