Wrongfully imprisoned Harry, Snape rescues

[Guest Anna]

I'm looking for a fic that I read a LONG time ago.

Harry is wrongfully imprisoned, I think it may be because he cast an unforgivable when killing Voldemort, although I may be mixing it up with another fic. He was definitely WRONGFULLY imprisoned though. He slowly is losing his mind in Azkaban; Dumbledore and Snape (I think, there may have been someone else as well). They see how non-responsive he is and Snape finds it very wrong. He somehow rescues Harry, I think it might have been a portkey or something. I think there was a scene where Harry is swimming in the sea when trying to escape and a mermaid helps him because he is so weak from his stay in Azkaban that he nearly drowns.

Harry manages finally to get to the safehouse that Snape has set up; I think it was a cabin in a wooded area and it was heavily warded.

Harry struggles with being free; I think there was something about him running outside (still inside wards) when Snape comes with supplies (I think in a basket) and to check on him.

I can't quite remember what else happened, I'm pretty sure it was still a WIP when I last read it

Any help is much appreciated!