It may not be so tragic after all..

Admittedly, the ending was a bit shocking, however, I don't think this is the end of their relationship. Mona seems like a surivor of would-be very devastating relationsip and left unforgettable impression on her. She is made of tougher stuff than Tamsin might have thought of her and is very resilent,as you can see in the scene where she's depicting her relationship with Ricky to Tamsin. The depicted relationship is terrible and can hurt someone susceptible a lot. But Mona is depicting it very casually and playfully. It also shows that she is leading the completely different life from Tamsin's.Maybe it was Mona who got the greater benefits from their summer love. They are both girls and can start afresh in next summer when Tamsin comes back, because they know each other well by this time. I suspect Tamsin is a very well-adjusted, charismatic and brillant student at school. She will steadfastly tread on the road to success and marry a rich and powerful man. Remember the Nietzche quote? On the other hand, Mona is emotionally needy and it is understandable that she easily falls for Tamsin who shows affection to care about her by buying her engine,bringing breakfast and etc.Even her name Mona is not her real name. It was what her brother used to call her. Mona was living in a imaginary web which other people in her life weaved around her. The pieces of Mona's life are so different from Tamsin's as to spark her interest.

As I keep re-watching this movie, I found out that Tamsin used sex to manipulate Mona. It was Tamsin who first kissed Mona while swimming and the look on her face while they are having sex is a bit undechipherable.It might be possible that she let Mona have sex with her to make Mona a perfect plaything.On the other hand, having sex with Tamsin was not a so self-conscious choice made on Mona's part.She just did it simply because she wanted it, without reference to her being a lesbian.
Of course, there are soft gestures and moments when Tamsin displays genuine affection to Mona, such as holding Mona's hand when they were taking stairs. IF she doesn't care, why would she bother to come to Mona sitting at stream?

Does Tamsin's personality have anything to do with her social class? People of her class don't display emotions or feelings so explicitly to the lesser ones whom they consider. She maintains this cold exterior when she first met Mona. It was when Mona started talking about her ex-convict brother that Tamsin showed interest in her. Class distinction mainfest itself in one's gestures, facial expressions, tone of voice, accents and the way he/she carries him/herself. Arrogance is the armour of the privileged class. Emily Blunt was excellent in pulling off such arrogance partly because she is from the class.

I hope their relationship go on as they get old and mature, even though Tamsin remains to be a conniving, manipulative bitch who takes advantage of everyone else for her own amusement and success with all her wealth, intellect and charm.
I don't think she's evil or a sociopath. She's just a child. Mona is a lot more mature and has gone through a lot in her life. Mona is the one who is really alive and feels. The relationship with Tamsin is just a part of her life full of turmoils. But she'll pass it as in the scene she's riding on the bike without engine. She responded by saying that "it rolls". Life to her is like that.