ONS's are only ONS's when you don't have sex with them more than once. It becomes casual sex when you do it again, catch 22
I hate it as well, but I felt the same way you did and then six months later, someone else even more 'perfect' would come along. Now I look back three girls ago and I think "What the **** was I thinking falling for
them?"
When you stop putting so much importance on one person and believing that you can be attractive to other people, then you won't get so emotionally attached and the whole thing will become easier.
No. 1 rule - Every new person you meet knows nothing about you. It's a clean sheet. You may look back at your past and think "Jeez, I'm a 'tard", but they don't know your past, so the least you can do is pretend to be cool and confident and nonchalant, and then you have less cause to be anxious.