The way I see it, Dementors and Boggarts are two different magical creatures (or non-beings) which act on humans in different ways. Where a Dementor draws on people’s happy feelings in a strong, but general and progressive way, a Boggart expertly and instantly identifies a person’s single most feared animal or human form, or object, and exerts its influence in a visual, literal way. This is why I think that Snape would be able to defend himself against a real Dementor either by launching a Patronus (we know from canon that he is able to do so), or by performing Occlumency (he will be able to shield his happy memories involving Lily from the Dementor, since they are very important to him and have formed him as a person since his youth).

In other words: with the Dementor it is a mental thing where Occlumency will work (unless, perhaps, if they attack very suddenly, and in large number – I’m not saying that Snape is invincible, and it’s true, he has lived through a lot of horrors). With Snape’s Boggart, however, which is in the very realistic, specific and material form of an accusing Lily, the attack is not mental but visual and direct, and Occlumency won’t work for Snape because the sudden appearance of the shape of Lily triggers his trauma of having lost her, in a different, direct way (as we will see another example of in an upcoming chapter).

Moreover, in this scene, the Boggart Dementor belongs to Harry, and the attack on Snape comes from Harry’s mind, not from the Dementor/Boggart itself. Snape is not affected by Harry’s Boggart, he is affected by the images of Lily that Harry forces upon him - that’s why he can’t get rid of the Boggart when it is in the form of a Dementor. Snape cannot Occlude against Harry in this scene, because he has already spent hours messing with Harry’s mind during their lesson, and Harry is desperate and therefore forceful.

I can imagine that Snape might be able to escape a Boggart by simply closing his eyes to break the visual spell, and tumbling out of the room – but that would not exactly be “handling” a Boggart, would it? In this case, Snape cannot flee the room, because that would mean to leave Harry at the Boggart’s mercy, so Snape is trapped in this scene, alternating between reliving Lily’s death through Harry and receiving Lily’s accusations through the Boggart.

What I wanted with this scene was to give the reader an idea of the extent of Snape’s grief. I believe him to be quite damaged by what happened with Lily all those years ago, and I would say he has some kind of post-traumatic condition. I’m sorry I didn’t quite succeed in conveying this – any idea how I could make it clearer?