I'm going to take a different approach. It's not that the general advice on focusing internally is bad. To the contrary, it's the best advice. But there's other things to do as well to help mitigate what I call "serial rejection response".
People tend to think that their attraction standards can't change. But they do, constantly, for plenty. They change as we age, as our perspective shifts as we gain experiences, or even just by repeated exposure to other standards.
Now, I'm going to venture into some shaky territory here, so be aware that there is disagreement in published information about some of this.
There are multiple things that go into attraction. They one thing that seems universal is symmetry. The more symmetric the face and body are, the more likely people are to find it attractive. It's a pretty objective standard too, with not much room for interpretation.
However, there's also signs of health and viability as a possible mate. That's where you run into the first thing that can shift. We don't actually have a great ability to read health visually. And there's subjectiveness inherent in what is and isn't considered healthy, and that can change easily.
A common example of that is acne. Not everyone views is as sign of bad health, but some do. It's also something that is more common in teen and young adult stages. When you're younger, and everyone around you is more prone to acne, you tend to filter it out because it's so common, we just don't see it as a flaw in every case. And there's gradations as to how severe acne is before an individual detects it as a negative rather than the norm.
Body build is obviously the same kind of thing. It's a subjective, and largely subconscious, "template" that gets built up over time to filter other people into categories of "possible mate" and "nah". But the very fact that it not only builds over time, but can change later in life as we're exposed to more variety, means that it's something that can be adapted to.
Now, you can actually consciously change what you're attracted to, though it isn't easy, and takes time. This would expand the pool of attraction to give better chances of mutual attraction.
But, once you realize that the vast majority of people don't know it can change, and that they're just drifting along responding to stimuli they don't even understand, it means you don't need to worry about it as much. It becomes a matter of patience in finding someone, or shifting closer to the local norm of what is and isn't attractive (and there is a large degree of it that is very local, down to town size and smaller; you'll find people at different schools in the same county having different standards as a group).
This helps remove any bitterness because, once the idea is internalized, you know that not only is it not you being unattractive at all (and everyone is attractive in some way, even if it's very limited), it's just not the right time and/or place. It's a crappy hand to be dealt, but not an insurmountable one.
You'd be amazed how just moving to another town can radically change how much attraction you receive. Just changing neighborhoods can do it in decent sized cities or towns.
I promise you, plenty of girls and women like extra skinny guys, the same way plenty like dad bods, massive muscles, trim athletic builds, or chubby to obese bodies. It's all about where and when you are. You, exactly as you are now, may find that women shift towards your body type as you age. Or you might not, but be aware that it isn't universal or permanent inherently. A super thin guy in his twenties is running around asking out women roughly in the same age group most of the time, and that could be a grouping that's locally only into dad bods as a majority. But they get older and change too, so you run into the ones that shift towards your type.
And, obviously, not only will your tastes change over time whether you want it or not, you could start work on finding the attractive qualities in a wider range of women. People think of this, and talk of this, as "lowering standards", but that's bullshit. That way of thinking assumes that any given set of standards is right solely because the person using the term thinks their standards are better. And, again, that's bullshit.
As an example of that, if I dig thicc ladies, but have no luck with them, it isn't lowering my standards to date someone that's model skinny, it's just a different standard. If I didn't like that kind of body type, I would have to work at seeing the good parts to change the "template" in my head that says attractive or not. I'm lucky in that I've never really had a type, but I do have greater or lesser attraction to different types. I have successfully changed that over time though, multiple times, partially just to see if I could.
Seriously, do a little thought experiment here. Find someone you don't find attractive that's with someone. Ask yourself what they see in them. Then look for it, because I promise you it's there. They're using their internal template, probably without thinking, and found a match to it. Once you start realizing that there are things in people you aren't attracted to that are attractive anyway, you start to look at looks in a new way.
This is getting long, so I won't bore you with anecdotes about my own life and how this works. But I will end with something to think about. Ever know an old couple? The kind that sum still kiss and hug, and make googoo eyes at each other, call each other beautiful and handsome. They exist. And they are attracted to each other, and likely always have been despite the fact that neither of them would have been attracted to the other fifty years prior.
Remember that, and you never need be bitter.