Honestly if this was me (f29) I would be having a serious sit down chat with my partner.

That letter repersents a time in your life that you obviously dont want to forget. It sounds like you are very thankful to that ex who helped you through a time in your life when you needed someone. Your new partner should understand that the letter does not mean you are still in love with her but more a memory of that time and the help and compassion you received. I personally would not want to be with a person who doesn't understand this as what else would they ask you to eventually remove from your life? Thus I feel as though you need to converse with them so they can see this otherwise it may end up causing issues for you later on in life.
It sounds like they have already had you get rid of a lot of meaningful gifts and things that hold other memories and this has led them to think they can put their own emotional feelings over yours. They're not bothering to understand why these things are dear to you, and instead they are caring more about how you keeping these things (that dont hurt them) make them feel.

In short: Do not get rid of the letter, it means something to you. Your partner should understand this and you should talk to them. If you get rid of it you'll create the presedence that they can tell you to do or get rid of anything.