First of all, sorry for the broken heart, you've proved yourself to be a guy of loving quality.
YES. . . . . YES . . . . . YES. . . . YES, YOU DID THE RIGHT THING, now let me land from my private jet

The girl has never been yours, all along while you thought you guys were dating, she wasn't yours. You were just dreaming and THANK god, you've finally woken up and i hope for good.
Believe me i know from experience that ripping someone away from their relationships is an A-CLASS mistake. And if you had continued with her, you'ld have turned into her slave because you wouldn't want to do or say anything that'll hurt her so she dosen't run back into the other guy's arms and you'll find yourself thinking it ok when she's with him just in the name of love. It wouldn't have been right. That's not love
If you'll be patint, let me give you my own true life story:During my secondary days, there was this girl that was best friends with this other girl, but i liked this girl soo much and wanted her to be my best friend, i wanted her to leave her other friend.
I feel so stupid now but i did everything for her to notice me, to the extent that the girl now fell in love with me (not romantically oo)
I showered her with gifts, my neighbour in calss who knew what was going on and knew my inentions now called us out of class one night (it was a boarding school), and said she thought it was best if the girl in question chose amongst i and the other girl, who she wanted as her best friend.
To our greatest surprise, she chose me. . . . . .BUT i can honestly tell you it didn't work out. WHY? cause she wasn't rightfully mine. They both later found back together and i accepted it with my whole heart though i was a bit hurt. but i knew deep inside me the friendship we shared was bought, not earned, it was a scam, not the real thing.
That day i believed in the fact that: what Godhas joined together, let no man put assunder.
My dear, it shall be well. And for your sake never think of getting back with her.Let her be, and very soon i'm sure you'll find your one and only. Someone who'll love you, and you alone. Someone you wouldn't have to share with another guy, someone whose heart is not divided.
Good luck and stay strong

Your best is yet to come.