Happy 7-months-ish relationship, Anonymous! I would like to begin by telling you that my first-ever girlfriend kept a journal for me during the summer that we fell in love. She wrote in it nearly every day, detailing so many of the moments we spent together, and at the end of the summer she gave it to me to keep. It was one of the most incredible gifts of all time… and it was given to me twenty years ago. I use those italics because holy shit how did I do *anything* twenty years ago, but also to underline this point: I am now married to an entirely different person and I still keep that journal in my house. It is a memento – exactly the word you used to describe the items in your question! – and a very powerful memento of a very important time in my life that I spent with another person who I loved dearly.
Keeping mementos of past loves, in my opinion, is a very normal (and vital!!) part of our lives. We are people, after all, so we don’t remember much of life’s detail as weeks, months, and years pass us by. When we share something important with another person, we grow. The people that we love and have loved are very big parts of who we are. So, your girlfriend holding on to milestones in her life does not mean that she is still in that place today – it simply means that the place she was once in was very important, and she wants to be able to remember certain pieces of it.
Do you have a right to feel upset and jealous?? Of course you do! We all have rights to feel our damn feelings, and finding those mementos stirred up a lot of feelings inside of you – you are jealous and upset, and you also explained that some of these feelings might come from the fact that this is one of your very first relationships. So, you are probably thinking things like, “If she has these things it must mean she wants to be with those other people still!!” (to which I would say no it most certainly does not and please reference my memento-discussion above), or “I am probably not going to be as good of a girlfriend as those other people were!!” (to which I would say: you most certainly *are* going to be a wonderful girlfriend, but you will not be the same as they were, nor should you wish to be – she is with you because you are different!).
My words probably won’t make all of those feelings go away, and I would encourage you to talk to your girlfriend about those feelings – but not in a way that makes her feel she has done something wrong. She hasn’t! You aren’t doing anything wrong by feeling these things either, but you should look to her to help you through those feelings. She might be able to explain to you why she keeps the things she keeps, or she might be able to just say to you, “Hey. I care about you right now, and that is what I want to focus on. I can remember things from my past fondly and not want them back – and I would love it if you could work to trust my feelings for you.”
Whatever you do, don’t ask her to get rid of those memories. Often, in love, the tighter we try to hold someone, the more they wriggle away. The most powerful way to love a person is to let them be who they are right now while also loving the person that they once were (and all that that entails) and the person they are becoming as each and every day goes by.