I'm not clear on whether your focus is on finding a relationship or finding deeper friendships and my advice is going to differ, based on what you are looking for.

In terms of friendships, has this been a problem for your entire life or did it develop at a certain juncture? How did you do socially as a child, in school, etc. Do you get along with coworkers, family members, older adults? Who, if anyone, are you close to? When you do try to make friends do they share your interests? Do you open up to others/ ask for favors or are you expecting others to engage with you on a level you don't engage with them (the inverse could also be a problem if you open up too much or ask for too many favors). Where do things tend to break down for you in developing friendships (how close do you get to people before it becomes clear you aren't in the friendship territory you want)? Is either your confidence or your attractiveness a factor (for example, it is hard for many people to open up to someone they perceive as perfect)? I can keep going with these, but hopefully you get the picture.

In terms of relationships I'm curious about how you are meeting prospective partners? What kinds of people you are dating? How you approach dating and conversations about what you are looking for? Basically what are you like as a partner and is the type of person you are seeing actually compatible with that?

I have a friend from highschool who is gorgeous, incredibly smart (went to ivy league schools for undergrad and her MD), is an accomplished athlete, has some interesting hobbies, and is a genuinely good person. I really love my friend (based on a shared childhood) and I also find her really boring at times. She has, unsurprisingly, struggled to maintain relationships with men or even close friendships. To be honest, part of it is her lack of warmth or reservedness. She isn't cold per se, she just is careful not to share about herself until we are a few drinks in (and honestly not until people are too busy having fun to want to hear about what is important to her). Part of it is also that she never takes initiative. If we are doing anything I have to suggest everything/ plan everything (even when I'm visiting her in her state). I'm never sure if she enjoys anything we are doing, even though she obviously asks to come along. Honestly we've been out of highschool a decade and I see her about once per year, we've travelled together and had fun in a group setting, but everytime I'm alone I find myself inviting others and focusing on others. It's not that she is unintelligent, she is incredibly intelligent, it's just that she isn't very engaging and is careful not to show a very human side. She once confided in me that med school was hard at times as though she was telling me her deepest, darkest, secret (and that was the most vulnerable I've ever seen her). Add to the fact that she is tall and only wants to date men who are similarly tall, athletic, and accomplished with no serious relationship experience and well, it's challenging for her at 30. Is it possible something like that is going on with you?

On the other hand, as others have said, is it possible you think too highly of yourself? Or don't think highly of others? Or that you are valuing an idea of what a friend or a partner should be rather than valuing actual chemistry? It's hard to know based on what you've written here. But maybe the next few times you go on a date or a friend distances them self you consider asking for genuine feedback/ confide that you've been having trouble? Or even consider asking one of the guys you've dated if you had a good enough relationship/ things ended well? Why do you want friends and a partner in the first place? Is it because you both have something to offer and want closeness (i.e. is mutuality a factor) or is it because you want something from others?

Finally, you'll only get used as a placeholder if you let someone. If you know you don't want to be a short term hook up or a friend with benefits don't be. Don't continue to casually date people you want more from.