Re: Not Proven, or as the Scottish lawyer joke goes, 'not guilty but don't do it again' in rape and abuse cases, which are often historic, predominantly committed without witnesses and where evidence is sparse or very open to interpretation, it can cause problems, as some victims don't feel it's any sort of validation, and as 'the middle road' it doesn't provide the necessary closure. As a PP mentioned, and as is evidenced by the joke, it can also stigmatise those who are given a Not Proven verdict. The verdict is set to be abolished. IMO that's a mistake, in some cases it is by far the most appropriate verdict. as it can reflect the uncomfortable shades of grey that colours much decision-making around prosecution and punishment.
OP, regarding your feelings, by stating you felt the prosecution case was lacking, you as jurors made your feelings clear to the victim. It's not something that happens routinely and it's a clear indication that the victim was believed, but there was insufficient evidence to convict. I agree with a lot of the PPs who have tried to comfort you, we can't criminalise people on the vibes, police and prosecution have a job to do to tip the balance from 'sounds like he did it' to 'he couldn't not have'. Sometimes it's an insurmountable task, sometimes individuals fall short, sometimes it's judicial bias, sometimes jurors hate the police, sometimes it's a miscarriage of justice, sometimes a verdict gets overturned on appeal - and sometimes it's just the way it is.
Additionally no-one who has their case heard by a court is allowed to believe that there is a certain outcome. They'll have known this could go either way, and will hopefully be better supported to deal with the verdict than you have been. One cog shouldn't hold themselves responsible for the working of the whole clock. You did your best, and can do no more.