Why do my feelings for my ex boyfriend come back after seeing him?

8 months ago I broke up with my boyfriend of 3 years and a half (my first boyfriend). It was very sudden and violent. Before we both wanted to marry, have kids together... and I had to go abroad for my studies (the first time ever we were apart for so long) and I started seeing life differently and realizing there were issues in our relationship that would not help us in the future (he had a very dominant personality e.g. we were doing things he likes but never the other way around), so it was better to break it off now then later when e.g. when we have a mortgage and kids.

I thought I had put my ex behind, and am pretty happy in my life now. I've been dating someone else for 3 months now and we have more compatible personalities, similar tastes/hobbies, and we both like each other and hanging out.

However, since a couple of weeks/few months, I’ve been thinking of my ex at seldom moments, and even more when he contacted me to wish me a merry Xmas and when we planned to meet up. I just saw him 2 days in a row (we do not live in the same country, but I was on holidays over there), where we spent time and acted like we were together again (e.g. holding hands and kissing - I know this is immoral as I'm with someone else. Please do not make any comments regarding this): this felt so familiar and good to be with him again, as if nothing had happened.

We had said that this is just a parenthesis in our life (the holding hands and kissing for 2 days) and we can't go back together, but we will always be special for one another. He looks like he has finally managed to move on and accepted the end of our relationship and its reasons (it was very violent for him this break up). He said that for his next girlfriend (he’s still single) he wants someone that has pretty much an opposite personality to mine. Its good that he is looking forward to advance in his life now and I only wish him to find happiness.

In those 2 days, I couldn’t help crying at moments (I had not cried once after our break up, only when he contacted me in December) and he would comfort me and take me in his arms. I feel like maybe I've made a mistake by having broken up with him and still love him: its like all my feelings just come back up and makes me cry more when I think of our old memories together. Is this just temporary and in a couple of days everything will go back to normal and I will stop thinking about him?

Another thing is that when we just broke up, he demanded me to give him back a ring he had given (but I couldn’t do that because of logistics reasons) but then didn’t mention the ring when we went back in contact. When I saw him I had the ring with me, but he started to cry (me too) and he told me to keep it. Does it mean that even though he says he has moved on, he still harbors feelings for me and if I pushed he would want to be with me again?

Thanks for reading my long text… It helps a lot to be able to write this down.