I offer this perspective with much care and respect, as I have, many many times, witnessed the devastation experienced by victims who try to have their cases prosecuted and are unsuccessful, for a variety of reasons. I myself have never had the strength or courage to even attempt this, in spite of (or perhaps because of) having worked directly in the field for a long time.
But it is important to try to remember that there are countless complicated reasons why some cases are prosecuted and others aren't, and only one of these reasons is related to whether or not you are considered to be telling the truth or credible as a witness. Unfortunately, much as they often want to, police can't simply put together a brief of evidence where one does not exist, or where existing evidence is significantly contradicted or compromised by other evidence, unwillingness of witnesses to give statements, or any of a host of other obstacles. I know how frustrated and distressed many police often become when they are unable to proceed with a case through lack of evidence or other legal impediment, because many feel the full weight of their perceived responsibility to achieve justice, and care deeply and passionately for the cause that they work for.
Obviously, there are lazy, incompetent, careless and disinterested police. They are just people and exist along the full human spectrum from good to bad to indifferent. Perhaps they are the police you have had the misfortune of encountering... or perhaps you are simply in the horribly unenviable position of having some evidence, but not enough for prosecution, or other conflicting evidence (which you may not actually be aware of) which has been deemed to make your case legally untenable. It is in nobody's best interests to attempt to prosecute a case with a low likelihood of successful prosecution, and a big part of this principle is based on protecting victims from the trauma of legal processes unless there is a reasonable likelihood of a successful outcome.
I am sorry if I have rambled or appeared to be taking sides in a situation I know nothing about. I just wanted to offer an alternate possible perspective on why your case may not have been prosecuted to date, and to reassure you that as difficult as it is not to do so, it's important that you try not to internalise feelings of being disbelieved, rejected or invalidated by the police as a result of their inability to proceed with the case.
Please, please, please keep the e-mail, or if you don't actually want it in your possession, give it to the police anyway, or to a safe person who can keep custody of it for you. There are countless reasons why this could be a security matter for you in the future, or a source of evidence that might, when combined with evidence yet to be produced, tip the balance in favour of a possible prosecution. RussH has made a critical point in that none of us know when or if our abusers may/might have struck again, and may perhaps make mistakes or leave evidence that allows historical crimes to be linked to current ones, and previously cold or unprosecuted cases to be reopened and actioned. You deserve to give yourself every chance at legal justice, protection and peace of mind, and keeping that e-mail is a small but critical step you can take in that direction.
I'm so sorry this has happened to you. May you take gentle care of yourself.
Maddog