So this story was a slow burn and build up, but not with a lot of sexual tension, just flashes. That isn't a bad thing, because you really got to see the development of the characters together as a couple. They became friends, and it wasn't driven by their lust, it was ruled by them actually getting to know and like each other. Then, every once in a while, both of them had a flash of attraction to the other, but it simmered with them, rather than dictated them
It needed to be that way because our characters were dealing with some pretty big issues and anything else would have soaked the story in too many emotions. Phoenix was tracking down her birth mother while hiding that from her adoptive parents. Justin had a mother suffering so deeply from grief she has been suffering for years with no improvement. Justin himself still has PTSD from his own grief and handling his mother
Phoenix and Justin find relief with each other, it blocks out the real world and they find comfort, happiness and an ability to be true to their feelings. While Phoenix eventually shares with Justin what she is going through, he decides it is too much and he is too messed up, to share with her.
Things work out in the end, and they are both committed to working things out, It isn't tied up all neat and tidy, but then again, what relationship is? They want to try and work things out, and are committed to doing that long distance. It may seem like an abrupt ending but it fits with our characters. They have lots to work through on a personal level, and decisions for the same, to be able to close everything up nice and neat at this point