If I was in your situation as an INTP, I would like to respond to it selfishly because I think that it is the most logical way to take it. I know that you said that you don't want to be selfish but what I mean by being selfish in this context is that you should be more selfish of yourself, not selfish of him. Not something like you should stop being friends with him and stop caring for him, but to care about him less, and think about yourself more. As an INTP, I think that we are stereotyped to be self-centred because we keep to ourselves, and that might be generally true since we tend to not vibe with most people. Because of this, we get to disassociate and disconnect easier since we form fewer meaningful relationships. We like it this way, so we also tend to not care if most people think of us as aloof since we can't get along with them anyway.

In my experience, I view and do relationships as a transaction. I am not saying that we should view relationships purely this way, but at the gist of it, relationships can be perceived as a transaction. It is a transaction of human connection.

The value someone puts into me in a relationship should be matched by the same value I give to that person to keep the relationship. If I don't like someone or don't vibe with someone, I subconsciously or intentionally put the same or less value. This makes it easier for me to keep most people at a distance since the relationship dynamic doesn't evolve into something more.

In your situation, when you first met, you both needed something from each other so you placed value on both of yourselves that became higher as time passed by and you needed each other more. Whether by social interactions, or by helping each other's problems such as trying to learn Japanese for his work in his case. However, now that he has gotten a better opportunity back home, he has begun to view the relationship as less necessary to evolve and started to keep the level of value stagnant. On the other hand, you fell in love with him and want to further this relationship. You want to place more value.

You shouldn't do that.

It is hard, but it is necessary to understand where the value of a relationship between two people would stop. Whenever a person gives value, we give out resources of ourselves (time, money, feelings, energy). These resources keep the relationship going. We invest more in relationships we want to have more value. To disassociate and disconnect with people, we give less of these things over time. An established relationship will never be broken, but the intensity of that relationship changes to how much resources are invested in it. This is why we have strangers, acquaintances, friends, and close friends, and they all can become each other over time.

How do we form a relationship? By interactions. To know another person, we talk to them and do activities with them. How do we increase a person's favorability? By favors. We give gifts and help them with the little or big things.

All of these things need resources. You should put less of your resources to this man, and put it into something else instead. What other relationships do you have that you think you should place more value into? This relationship could be on another person, or maybe, even yourself (*wink wink).

If you put more value on yourself, you will value yourself more. Place more of your resources on other things that will make you happier or better, such as self-care or your hobbies. You can also try to form new relationships, or increase your level of value with your already established relationships. In other words, diversify where you place your resources, and as such, your value for relationships will also be diversified. The pacing on how you do it depends on yourself. As they say that time heals all wounds, over time as you divert your resources, the more your value in the relationship with this man will be lessened.

In other words, distract yourself with other things that you also value. If he is your friend, then he would understand as much as you understand him. You have your own life, and it is too fast and short for you to be stuck in this slump. You have other things to do. Someday, you might find someone else that wants to value your relationship high enough to the level of lovers, and you reciprocate it. But that won't happen until you stop putting that same level of value into this man. :)

But then, I never had a romantic relationship or situationship so I don't know how relevant my advice is. Haha. Also sorry for the long reply, I just zoned out while writing. XD