We were together for almost 10 years (on and off for bits of it). I met him when I was 18 and we went through most of our twenties together. I loved him so much and still have love for him still. But we lost the "in love" part. In the end, no matter how much we tried there was a disconnect, physically, mentally, emotionally. My family loved him and his family loved me, and I think when we were all together was when that felt the happiest. But when we'd get home, I was lonely. I picked up a million hobbies to do on my own and he didn't want to be a part of them. The only thing we shared was TV shows and video games, but there was no actual depth to it. All we had was our history together and that we got along really well and loved/cared for each other. Anytime I mentioned the future or marriage, he didn't seem interested. He also liked to drink and would become a completely different person when he was drunk. I tried to talk to him about it and he didn't believe he was starting to have a problem. I could never see my life without him, but I thought about our future and realized that this relationship wasn't good for either of us long term and that we couldn't simply stay together just because we were scared of change or being alone. He is a kind, wonderful, loving, caring, funny, intelligent person and I'm grateful to have spent so much of my life with him by my side, but in the end that wasn't enough. So I let go. It was devastating and heartbreaking and tested all the courage I had in me. I knew it was right. I knew I had to. That doesn't mean my ex was terrible, it just means we grew to not be right for each other anymore.
Not so long after we broke up, by strange circumstance, I met the man I would marry. I could go on and on about how wonderful he is, but that's not what this thread's about. I called my ex (we were still friendly through the break up) to tell him I was in a serious relationship (I wanted him to find out from me instead of someone else) and he wished me all the best and was grateful that was the news I called with instead of trying to get back together (which is what would always happen in the past). He thanked me for being honest and we're still friendly to this day.
It was weird to lose someone who I had spent so much of my life with and who knew me so well. I grieved for a long time and sometimes guilt and grief still come back, but in the end I know I made the right decision. I know I'm with the right person now.