In my life I have noticed that I have a very hard time with letting go of people and things.
Here are some reasons that I have come up with to why letting go is so hard and painful and something is almost impossible to do:
If you invested a lot of your time into something or someone it can be difficult to want to give up. You start thinking to yourself that if you've hung on for this long why not try to make it work? You don't want your efforts to go to waste.
Broken hearts are messy and no one wants to deal with the emotional aftermath. If letting go means having to deal with this then fuck that. No one wants to hurt voluntarily or involuntarily.
You've been hurt enough in your life and you're just sick of crying. Letting go leads to heartache and heartache leads to bawling your eyes out at random moments of your day. Crying is exhausting and we tend to avoid it and not embrace these emotions. If we let go then the tears are gonna flow over.
Letting go of someone can leave you feeling lonely. For instance if it's a relationship you don't want to lose the person you love because you feel like you're gonna die alone. Or worse if it's a friendship you don't want to not have a best friend that you can go to. No one wants to be alone in life so this fear is only natural.