for some reason can't hold relationships like others

I have asperger, not a strong autism but still makes some things different for me. Also I'm an introvert, if I'm in my room I can go days with low human contact, tinkering on my projects and things.

I'm spending my summer abroad working as a waiter, while also being in a relationship. But one of the problems is that my partner is in my home country..
And while it works well when we are near each other, I feel like I can't maintain proper relationships, if the other person is not within a reachable distance.
I don't miss people how I should, how others do it. I don't miss people in a loud constant way. They just sometimes pop up in my head and I miss their energy like their personality. But this is never like a constant feeling.

Currently as I mentioned I work as a waitress, usually 6 days a week in 8 hour shifts. Some days I even need to be in overtime. I did notice that this is not exactly the job for me as I have to deal with humans constantly, like be present and things. I also have flatmates here, who want to have conversations with me and things.
And while I would have the time to give attention to my partner who requires it (and I feel that as a totally valid need coming from them), somehow I just can't. I can't quite reason why, but for some reason I just feel incapable to do so.
Logically, I know that their need for my attention is valid and real.
I feel bad about this all, and I don't exactly know why I feel incapable.

i gave this a quick read-through and just wanted to say that you are valid regardless of what you may think of yourself.  The thing about romance is that there is no " real " definition of it; there's no guide to follow on how love should be shown or how it should be done. While yes, their need for your attention is valid and real, so is your need to keep a distance. In no relationship should you feel like you're being pressured to love your partner in a way that's common in other relationships; you might feel incapable simply because the media have formed love so that it has to be exactly like what they show in movies, or else it's not real love. Most of my advice stops here unless you would want to continue discussing this, you might have luck speaking to your lover about the manner or to your roommates if you feel comfortable enough to tell them. There will always be someone out there who will understand you the way you want to be understood, and there will always be someone who'll be more than content to lend an ear about what you struggle with. :)

if it helps to hear, you're not alone. i relate to most of what you've said here, i'm pretty much the same way. i rarely miss people and when i do it's typically quite muted, i could go a very long time never interacting with ppl (especially irl), keeping up with relationships of any kind can sometimes feel like a chore bc most ppl want/expect way more interaction than i can or want to engage in, etc. overall i much prefer to be alone than around others, even the ppl i care most for. i rly do understand

i'm also autistic and an introvert and while i think that both definely play a big part in it, i am also schizoid (szpd, not schizophrenia)- which i only bring up as it may be worth looking into for you as well. there is also the concept of "emotional permanence" which can be made difficult by a number of various disorders and disabilities. i know i struggle with it for sure lol

not trying to armchair diagnose or anything ofc, i just know it helped me to put words/names to what i experience. and you may find helpful resources or communities even if you don't think that descriptor/diagnosis is exactly right for you 

i hope this wasn't a weird response! please lmk if i made you uncomfortable or anything, that wasn't my intention at all. i wish you well :3