I think Snape was completely obsessive in general, which, in particular, made him consider Lily Evans his only love ever. I don't use the term "true love", because that is up to much discussion, especially when it comes from somebody so obsessive. Nevertheless, here are a few thoughts to support what I say:
- First and foremost, he falls in love as an adult with the first little girl he met as a child. Yes, it is a lovely story. Yes, it happens even in real life. And yes, those are rare exceptions, and are usually accompanied by the presence of suffering friends and relatives who fail to make the guy understand that he's obsessed, that the situation is not healthy, that it's not going to happen, etc.
- He was determined to find similarities between Harry and James (in terms of personality), even though there was evidence to suggest that it was not the case. Or at least he's determined to treat Harry as if he was James, so he could avenge himself for past events. During Harry's first year at Hogwarts, when Snape reports to Dumbledore that Harry is arrogant, lazy, mediocre, attention-seeking and I don't remember what else, the headmaster tells him that he sees what he expects to see, and also that "other teachers report that he's charming, likable and reasonably talented" (perhaps not an exact quote). This appears in the last book, in the chapter "The Prince's Tale". Even Sirius seems surprised when he says to Harry "you're not much like your father". This behavior of Snape doesn't change over the years, even when Harry had proved himself normal ("as normal as can be expected under the circumstances") and had already faced Voldemort more than once, not to mention all the suffering he endured.
- Even at a very young age (before school) he had some extensive knowledge of dark arts and of other aspects of the wizarding world that are mostly the concern of adults. I'm not blaming him (after all, he does have a harsh background story with his family), but apparently his circumstances made him miss the happiness and care of a regular childhood, which is a perfect platform for personality disorders in adulthood. Remember when Harry sees James in the Pensieve, and he immediately notices the difference between the kid Snape and his father, because the latter "had the unmistakable marks of a child that was well-cared for, even adored" (again, maybe not the exact quote).
- When he starts teaching DADA in 6th year, he imprints on the classroom a very personal (dark, creepy, frightening) style. Curtains drawn, the air is closed, etc., just like in the dungeons (particularly like his old potions classroom). It might be part of his role-acting as an agent for Voldemort (he needed to be credible for the Death Eaters, so he needed to be dark and creepy and favor Malfoy, etc.), but I believe that part of it was a personal, conscious decision. It was his style because he felt comfortable with it.
- His notes in the potions book denote, yet again, obsession. For example, there were many marks and corrections for the spells he was inventing, and also obvious attempts to perfect even the simplest potions (for example, by adding a "sprig of peppermint", which counterbalances the side-effects of excessive singing and nose-tweaking, as Slughorn tells Harry after examining his elixir to induce euphoria). Not even Hermione "miss perfect" Granger did something like this, though we can attribute this to her narrow-mindedness and her strictness when following book instructions.
As for your final question:
Is there any indication he ever tried to find any kind of balance or happiness in his life? Or, with Lily gone, did he feel he had lost any chance for anything good in his life?
I daresay he did find something similar to happiness when he looked Harry in the eye just before dying, because
- he knew he was departing this world (so it was his very last chance to let him know about his feelings somehow, which is different to letting him know the facts (which he did with the memories)),
- he was seeing the eyes he so much liked, and
- he was side by side with the boy he "had grown to care for, after all".
Also, he seems to have tried (and succeeded) to maintain a certain balance in his emotions, the evidence being the doe patronus he casts in front of Dumbledore, who is so surprised that he asks "after all this time?", to which Snape replies "always". He actually needed that balance to be able to perform occlumency to perfection, because he needed to "control his emotions" and "discipline his mind" to hide from Voldemort the contradictions of his thoughts that were consequences of his role as double agent for the dark side and the Order of the Phoenix.