Darling, I dont really want to try to tell you how to live your life, but this is not a good thing.
It's normal to be a bit jealous of the people close to you, it's just human nature.
However, being scared of being left alone and feel that you need someone to rely on all the time is a bad thing for you and for the people around you.
This is the type of mindset that people who are in toxic relationships but can't let it go have.
The person is so scared to be single that they will accept all kinds of mistreatments and abuse.
Also, you don't need to be scared of your friend/partner or whatever finding other person and leaving you.
If they will leave you just because they met other someone then that means that they don't value enough and we don't need people who don't value us by our side.
If they want to go, just let them go.
If someone doesnt want to be with you, keeping them by your side will just hurt both of you.
It's better to find people who really want to be with us, instead of have anxiety and being obsessed with the possibility of be left behind.
omfg my childhood friend is like this too, she never wants me to leave her side or go out with others...