I don't know exactly why, but this is definitely me. On my best days, I can attract and get with women stupidly easily, but my success rate at actually maintaining them long-term is truly absymal. On the flipside, I know people who've only been with 2-3 women, but ended up getting married for some significant period of time. While it is true that attracting and maintaining interest are seperate skills, I'd like to point out that just because someone is in an LTR, that doesn't mean they're in a good relationship. For every man who is in a healthy LTR, there's probably 10 who are in a completely miserable one.
In any case, for me personally there are probably a confluence of factors as to why I struggle with LTRs specifically. My upbringing from childhood all the way through adolescence was pure abject misery. For privacy reasons, I don't want to get into this any further... but take my word for it. The point is, I suspect this upbringing resulted in an avoidant and at times insecure attachment (among other things, ie depression). As an example to demontrate this, on a few occassions I've even doubted the sincerity of the relationships with my friends and family (though not for baseless reasons I might add), so maybe that may clue you in do how bad it is. My "circle of trust", as you might call it, is brutal, with the innermost circle being surrounded concrete walls that are 40 ft high and covered in electrified barbed wire.
The other factor is i'm rather a rather peculiar person with unusual values, goals, sense of humor, lifestyle choices, etc that are often at odds with the sort of "typical" characteristics seen in women. One such example is the fact that I am an uber-introverted, and an extremely sensitive sleeper. I hate sleepovers with a passion for this reason, and this one example on its own has caused a lot of friction in pretty much every relationship I've been in, despite my numerous attemps to tactfully explain why I do not want to do sleepovers. And the introversion runs real deep - I do not like living with people, I do not like being around people for extensive periods, and I do not like people in my home for too long (1-2 days max). Unsurprisingly, I am completely uninterested in ever getting married or having kids. The problem is that this preference obviously is problematic even for simple LTRs, since cohabitating or at least spending a significant portion of your time with them at home is kind of expected. I simply cannot and will not do it. Also, I should mention that I do not think any of this is rooted in my attachment issues or psychology in general; I strongly suspect my introversion is innate, it's just how i'm wired. Same could be said for my sleep preferences.
Yet another problem I can identify is that I honestly struggle with the idea of commitment in general. I've never cheated, or "monkey branched" and I do take that seriously because I think both are incredibly cruel...however I do often find myself thinking about being with other women after only a few months of exclusivity. That is not my default mindset though, if I find the right girl I won't suddenly start thinking about other women. In my last relationship for instance, they primary reason that I started thinking about other women is because the sex was really bad, bad enough to override all the qualities in her that I really liked. And I'm not even a particularly sexual guy, my libido is quite low, but I still think sexual chemistry is huge.
There are other smaller factors such as being prone to bouts of depression and/or insomnia, or my near-chronically low libido, that probably aren't helping either. I also have historically been pretty bad at vetting women and ignoring my gut instincts, and I say this only to point out that some of the women I've dated in the past were simply not quality women (which is still my fault but I disgress). As I've gotten older, my vetting strategies and learning to trust my own intuition is probably the one thing I've improved on the most in the context of dating, but I thought this was worth mentioning.
If I had a normal upbringing and was a psychologically healthy and happy person I would likely have had far more healthier and longer-lasting LTRs, certainly would have run through more women too. I know this because in times in my life where I do feel psychologically "healthy" for lack of a better term, I end up doing very well with women, but it never quite lasts because at some point my whole world ends up crashing and burning and I have to start over.
In short, I would say 95-99%, maybe 100% of my problems in LTRs is from my "inner game" as they call it. It's not a lack of knowledge or lack of understanding a women's nature, it's the classic case of being my own worst enemy. One silver lining is that since I have zero desire to start a family or get married, I am not burdened with any sense of urgency to get into an LTR. As such, it's an immense weight lifted off my shoulders, and it lets me focus on living my best life on my own terms. The alternative would be doing what many other men do: letting all their dreams die just for the sake of starting a family and never becoming self-actualized.