I notice a trend in most of such stories. The FL is a virgin and that ML is someone who have been sleeping around. This would be frown upon and I find it disgusting in real life. This may be fiction but in real life is not uncommon. Why would someone so loose deserve someone who is innocent? As least, physically. ┑( ̄Д  ̄)┍
If someone able to let you go easily, it only means you are not that important to them. The more in love you are, the harder to let go. Even if you are miserable in the relationship, but the thought of losing that person is worse for you. That is why sometimes, people cling onto a hopeless relationship.
They could have been happy together but miho felt betrayed and started acting out wanted to hurt uke like how he is feeling. Uke is someone who fear loneliness and easily get attached to someone that shown him kindness and attention. And because of that, miho constantly felt uneasy about their relationship and hence, in order to protect himself, he tried to distance himself from uke and went back to his old lifestyle. This hurt the relationship even more and things starts to went down spiral. After a long painful period, uke finally had enough and decided to end it after realising he had fallen in love with someone else. To be fair, Miho is also suffering but was unable to let go of uke. As a result, they let on this toxic relationship longer than it should.
Hmm.. so ML just want the thrill of manipulating someone into being serious with him then throw them away? He was never serious about anyone? I am confused.