Does time really heal?...Who do I chose?..

I’ve been having a really hard time lately with my love life. I was with this one guy (I’ll call him M) M & I were together for 3 years and we’ve been broken up for close to a year now but I’ve tried so hard to move on and we always find our way back to each other. Either I’ll message him or he will message me. He still makes my heart do back flips, and I just feel so content with his and comfortable. We broke up because of lots of reasons. He stopped putting effort into our relationship and started disrespecting me by doing things I wasn’t okay with & he knew that. So I left. Well we tried to remain friends but that don’t work well when there’s feelings there. I ended up meet a really nice guy (I’ll call him Z) Z & I clicked right away. There weren’t butterflies or anything like that, but just comfortable soon as we met. He asked me to be his girlfriend and I of course said yes. We’ve been moving kinda fast and it’s making me nervous. He’s a really sweet guy. Well M and I still talk because of the whole trying to be friends thing but M wants me back, he’s been showing me effort. I feel like Allie from The Notebook when she finally thought she moved on from Noah & was gonna have a future with other dude, but as soon as she found out where Noah was went to him, cheated on other dude, and chose Noah. M & I hung out the other day went had a really nice day on the beach, he took me to dinner, but we also kissed and I felt so guilt for wanting him to kiss me. My heart just feels so content with M. I’m now so torn between them. M is the one love I’ve never been able to let go of, and we always gravitate back to each other. We also have a passed together, but I still think he’s a good guy. He always treated me good. Z is new to me and I don’t love him but he excites me & makes me feel comfortable. My family really likes him and would be upset for us to break up. I’m stuck. I don’t know what to do. Or who to chose. I’m happy with both, and know my future with either could be great. If you were in my position would would you chose? Or maybe someone has advice to give? Or has been through this.