How to get my ex back that i love?

i was with this one girl who i was blown away by she was everything i could of asked for but i was to selfish and immature to realise whilt in a year and a half relationship i messed up now about a year down the line i still have strong feelings for her i carnt stop thinking about her but i think she has a bf just wondering what other people have done in this position i know i am in love and shes my soul mate