Am I overreacting? Or do I have to accept that most people I get close to will walk out on me

maybe leave it for a couple of weeks, and if she hasn't contacted you in that time, then contact her again with an invite for coffee, of something, and then if she says yes, all good, and if she says no, then back off and give her space

To me it sounds like you are trying to get some unmet needs met by these relationships. I don’t mean friendship needs, I mean deeper needs. Eventually that probably comes out either in communication or unconsciously.

I have had a few friendships in the past where by the friendship started to go way beyond friendship levels towards almost an unhealthy dependence level, one ex friend was ND and another with significant childhood trauma. Eventually even though they were genuinely nice people I had to pull back for my own mental well being. A mutual friend explained the issue to one of the ex friends and they were completely oblivious even though it was a long standing pattern for them in friendships.

If this is a pattern, then yes, I would suggest you'r eeither doing somethign wrong or misreading relationships. how long was this person your friend/? True frienship takes time. you can bond with someone and have a real connection but not actually convert that into genuine long-term friendship. either because long-term you're not actually compatible, or because life just doesn't work out that way eg because of work/family commitments, distance etc. I still feel sad that a woman I became friendly with when DS was a baby moved away. I feel like if we'd stayed local we'd have become good friends but by the time she moved, we weren't close enough. It's no one's fault, it just is.

So after reading your other post, you say you have close friends who you love dearly. I suspect the fact that you can't get another friend out of your head does sound like you're in love with her and this is why you're feeling empty. Are you in a relationship that you'd actually rather not be in? And this twin thing you're looking for is actually another woman romantically?

Is this the online friend who isn’t coming to your birthday?

This is not something that I think normally happens, kindly I am wondering if there's a reason if its repeatedly the same thing over and over. Can you explain a bit more. How often are you messaging, calling? Are you wanting to meet a lot. Are you talking negatively a lot? Sorry just trying to unpick what it could be.

Now I have to find someone similar to not feel hurt and to cope

That's a big red flag to me. It tells me that you are seeking people for your own benefit rather than wanting to develop close connection with someone which is always give and take. Agree with the PP, you need to reconsider your motives and how your behaviour influences the relationship. And seek some therapy to resolve your personal challenges before attempting to seek closer relationships.

With kindness, there is probably an element of self fulfilling prophecy and self sabotage at play.
You are fearful and expect people to leave and then in an unconscious way you either push them away or pull them so close that they get uncomfortable and leave anyway.
Have you had therapy to work on your past issues and help you manage unhealthy relationship patterns?
Also, in what timeframe are you trying to develop ‘a super close bond’? True friendships that are super close take time (years) to develop. Relationships that get super intense in a short period of time rarely are long lasting as there is something toxic/codependent about them.

sounds sad, and you are probably just unlucky. I hope your friendships continue - sometimes there is a bit of ebb and flo, but the friendship itself remains solid underneath