When I met her, she was in an unhappy relationship with a rather controlling boyfriend…the old “I decide who your friends are” type. I didn’t get with her for a while once she ended it with him, however I do believe I was a big part of that decision.

She probably latched on to you, although you may not of realised it. You was probably great emotional support for her and she built a bond with you, unfortunately it wasnt good enough from her side to really form as strong a relationship that you thought you had. Thats not your fault though, thats hers. She obviously didn’t work hard enough within the relationship and just opted for the grass is greener theory (which is total rubbish by the way)
On the up side you have met your new partner and it sounds like you two are happy and well bonded together. Karma works in mysterious ways, they say.