You're welcome. Please remember that this ONE relationship not working, does not imply in any way that you are just bad at relationships or none will work. It was one specific person and it sounds like that person isn't good at maintaining relationships. It takes two. While I think looking at what you can do better and taking accountability are great qualities, some times you also have to acknowledge that the other person just didn't hold up their end and you can't be both people in the relationship. If this same thing happened in 5 relationships, maybe I'd be more inclined to think you were doing something wrong. Right now it sounds like they were.  

There are billions of people in the world. You don't want to pick just anyone one you can grab and do anything to make it work. You want to find one that is worth making it work. One that works with you. You might have some bad relationships until you get to that point and it's okay. This one relationship didn't work. No other relationship will be exactly the same as this. Hopefully future ones will be better. You will be okay and you will find someone better and when you do it will feel easy to keep the relationship and you won't have that fear because they'll do things that keep you feeling secure and you'll keep them feeling secure. 

I think your fear of losing the relationship came from the fact that it just was never really that strong and this other person may have always sort of had one foot out the door. When someone loves you and wants you to feel good and really wants to be with you as much as you them, you won't actively fear losing them because you'll know they'll stay.