AIO My boyfriend keeps mistaking me for his ex

My boyfriend and I have been together for almost two years now and there have been a few occasions where he mistook me for his ex. For example we wanted to choose to watch a movie and he was sure we already watched the one I picked but turns out he watched it with his ex. Not that big of a deal right? Can happen to anyone but lately it’s getting too much. In the past for months we’ve been wanting to watch Oppenheimer together and every time I bring it up he says “didn’t we already watch it together?” And I have to keep reminding him that it wasn’t with me, it was with his ex to which he always responds with “ohhh yea right. That night she almost broke up with me” and laughs about it. Idk how that can happen so frequently. But today topped everything. He was visiting his mom and getting some stuff. When he got there he sent me a photo of a love letter and texted me”look what I found! That was so sweet of you, remember?” To say I was mad was an UNDERSTATEMENT. That was a love letter of the same ex he always confuses me with. I told him that and he was adamant that it was mine. I told him that is NOT my handwriting, that I would never write a love letter professing my undying love for him after 2 months of dating (we had a bit of a difficult start because I felt uncomfortable with the words I love you when we just started being official) and the letter was full of kisses. I never kissed a love letter that I wrote him. But to top it off it ended with the words “I love you, your Sweetness” That is the nickname he gave ME. He gave his ex the same nickname he gave me and in my language it’s not really that common of a nickname for a partner (idk if it’s different in English)

I told him to give me some space to think. I always had a small hunch that he still might be into his ex but he always denied it.

Am I overreacting?