Advice for living with a non-minimalists

I would like to start by saying I'm not a hardcore minimalist. I do my best to only buy/have what I need. I need to have a logical reason when purchasing something new. I really enjoy having everything tidy and each item has it's own place. I realized my anxiety went down when I started doing this years ago.

Now I live with my partner who describes his childhood as not having a lot of money. He keeps EVERYTHING!! He still has letters his ex wrote him when they were in high school. (They were kind of high school sweethearts and got married really early. They have been divorced for about 8 years). He's 35 now. To clarify, it doesn't bother me he has letters from an ex. I'm pretty sure he doesn't remember having them. I found them when i was cleaning. I want to share this example of how he does keep everything. He has several coffee mugs, but he only uses the same three. He also never throws away his mail. This is probably my biggest pet peeve. I looked through his pile of mail (with his permission). He had cards from his mother, and at least two save the dates that were never opened. One of those save the dates was for a wedding that had already happened.

We have two different relationships with items. At times it can create some tension. Is there a balance here that we can create? I think both of us will have to compromise. Any thoughts or advice on that?