The Situation David has to fix is that Annabelle is not yet his. Despite his letters and calls, in which David confesses his love for her, Annabelle is married to another and does not share his affection. This does not stop David from imagining their life together. Indeed, on weekends when he returns to their home, he pours two martinis and imagines her there with him.
In This Sweet Sickness (1960) Patricia Highsmith's psychological thriller, she has created a fascinating character in David. Readers begin by empathising with him, feeling as though he has a crush that he will get over. But as the novel progresses you realise how deeply disturbed he is and how he has crafted the identity of his alter-ego, William Neumeister, to be all the things he is not as his mind unravels.
Of course it wouldn't be a Highsmith novel without a crime and David seems to get away with his, taking increasing risks and telling more lies until he cannot seperate fact from fiction. One has to feel for the women in David's life: Annabelle tries to let him down gently and encourage him to direct his affections to Effie, a local girl who is interested in him. David's obsession becomes more irrational and creepy as the novel progresses.
I really enjoyed this book. If I had one quibble it would be that it peaked too soon with the crime which exposes David/William's double life and loses a bit of momentum before the rapid-pace of the final chapters. But ultimately this is a fascinating character study which gets under the reader's skin.