How should I say goodbye?

My ex (22F) and I (24M) dated for a couple months and broke up. After the break up we continue to talk and sleep together and other stuff we used to do. She’s slowly been treating me more and more like she’s lost interest in me, even tho she says she hasn’t. She barely replies and takes so long to open my messages. I am in love with her and she says she loves me but I can’t tell if that’s true. Lately the situation has been bringing me down and damaging my mental health. I decided it’s best to say goodbye and give it time to maybe come back down the line and be friends or try again. I wrote her a letter explaining how I feel and saying goodbye but I also had planned on explaining the situation in person then giving the letter. I didn’t know if I should just give her the letter and say nothing or explain it in person and then give her the letter? What would you do in the situation and why? I’m leaning towards just giving her the letter and not saying anything.