Why do men stop wanting to have with their longterm partners?

My ex-boyfriend and I were together for 7 years and it would 8 years this summer had we not broken up. With the exception of a couple of those years, I have always wanted to have more often than he did. Nothing crazy, he just was never as interested as me. Probably because he had other options, which I didn't, because I only want with someone I love. Regardless, my mom died this summer and we had twice after that and it stopped. I went through a lot of depression about both and kept asking him about it and he kept making excuses (mostly that he couldn't physically do it, if you know what I mean) but saying he still wanted to be with me. Eventually, after five or six months of no , we ended basically because he was so sick of being asked and I was sick of low self-esteem. We still talk all the time and hang out. He still flirts me and I still flirt with him but it has come up and he still claims he has no ual desire. He is on two medications, one of which is Lithium, but I don't know. I love him a lot and he is the only man I have ever enjoyed sex with and ever loved. I met him when I was 18 and he was 27. I guess my question isn't how to get him back or if he is sleeping with others...I have already drawn conclusions about that just because true or not, people have to move on and part of that is accepting that actions speak louder than words. My question then is what did I do wrong? I asked him this question and he was adamant about how I didn't do anything and it was him. I kept asking and he kept repeating it, even though I told him that he would be helping me to just tell me what it was so I didn't make the same mistake with someone else. He still insisted it wasn't me and it was him and his ED or whatever. So, what causes me to lose interest with sex with women? I already know I screwed up by taking away the magic and not letting him be the aggressor - but I became quite emotional, paranoid, and demanding when my mom was dying.