Probably one of the most difficult parts of a break up is figuring out that you have to learn to be alone. You have to become fully capable of your own happiness. You have to pay for your own meal and order your own delivery (what???? no one told me I would have to do that!!!). You watch movies on Netflix by yourself and you go on double dates without a date and you kick yourself in the butt every time you see a mistletoe. Finding yourself and how you can make yourself happy is one of the most difficult parts of a break up. But it also makes it the hardest part of getting back in to a relationship.
Once you become the only person you depend on, it is hard to willing give that up to someone else. Like hey, I was doing just fine before I met you. What makes you think you can come in here and mess that all up??? Although a relationship doesn't mean giving up your independence, it does mean that there is someone else there looking out for you and someone you're obligated (but not entirely burdened by) to care for unconditionally too. When you've been getting along on your own for so long, it is hard to accept that someone else might want to take on the role of taking care of you too. On top of that, it is difficult to accept the role of doing the same for that person.
You do not have to be "tied down" in a relationship, but there are rules to follow. Like that boy you met six months ago at the club in a different country? If he ever magically reappears, you can throw that possibility out the window! The what-could-have-been's had no longer have potential. It's like throwing away leftovers. I probably won't come back and eat them before they go bad, but they are here for me just in case. Don't tell me that you're going to just throw away your half-size Olive Garden you ordered last Wednesday. It's not a bad thing at all to get rid of those half-ass relationships, in fact, it is usually a really good thing to get those out of your life. But the ability to let them go is harder than it may seem.
On top of all of this, there is still the fact that you've been through enough with previous relationships. If a falling out with someone who used to be everything to you isn't hard enough, the possibility of going through it all again is terrifying. If you have ever been in a toxic relationship you know it is not an easy thing to recover from. Constantly being taken down by someone who made you so vulnerable is one of the hardest things to get over and move on from. No way in hell will you do anything to possibly allow that to happen again. No matter what happened in previous relationships, it didn't work out for a reason. And repeating that wouldn't be any easier the second time around.
There isn't anything easy about committing to someone. Relationships are hard and they are scary, especially for someone who doesn't know what a healthy one is like anymore. It is important to be patient and understanding to those who aren't ready to commit. Just because you aren't scared doesn't mean someone else can't be.