holy shit she's extreme! She is lying to you, and whenever she doesn't win the heart of spanish Alejandro or whatever his name is, she comes running back to you. Of course there was a 3d party, there was a whole freakin fiesta.

"Her last entry was dated August 11th, about two months since we started seeing each other. She would write how she 'took a leap of faith' and fell flat. But that she still loved Eloi (despite his smell and dislike for cuddling) and if only he would give her, them, a chance, she knows they can go far in their relationship."

I mean, even if she's lying, which she's not - she'd still be writing journals about how she loves Eloi (so not Alejandro then) and how they could have a great relationship. So even IF she was lying, it still would be something wrong! You did not waste the best years of your life, why do people say that. How could it be a waste if you loved her and loved your years together? It's a waste because you won't get married and you need to find someone new? That's wrong too, relationships come and go at any age. You had a good time together but she probably got bored and adventurous, any girl who hints about being "spontanious" means exactly that: they want something more spontanious. Perhaps she needs to grow even more and experience more, that she wasn't ready for you just yet and you CAN do better than her. If you two got back together, just imagine if this would happen all over again and how she may keep lying to you.

If she was serious or worthwhile, she'd text you when she said she would, she'd ask for forgiveness and be honest, she WOULD NOT write in her journal her thoughts that so conveniantly matches your history (whenever she hates him, she contacts you, and when Eloi is available, she forget you). It's normal to want someone when you are left alone, like she was with Eloi, but understand that she only needs you in the manner of filling his gap.

"My head says to run but my heart is saying wait for her further explanation and maybe it would all make sense."
I know we should all listen to our hearts, but the thing is that our mind filters what our heart has to say. It's not your heart that's waiting for explaination and miracles, it's your 5 year old relationship who's become a habit to you in every way. Any "heart" will have a soft spot for an ex we loved but love is never about lying, doubt or being the second choice. I guess you can take your time and wait and see, but whatever happens - do realise she's lying to you. She is who she is, you can't do anything about that. Accept it.