Sometimes I feel like the family screw up

I dont know where I went wrong in my life. For the most part, things are good in my life. I have a good career (law enforcement), did 4 years in the army, a nice house, a couple offroad vehicles, a decent amount of good friends, im healthy, etc. But one thing I always struggle with is keeping a relationship and its getting embarrassing. I see my parents who were HS sweethearts and stayed together all this time no divorces, infidelity anything like that i mean in 30 years I dont think I every really saw them fight or anything. Then my one younger sister is married to HER highschool sweetheart and they are happy, had twin boys and life is good for them too. My youngest sister is in college and I dont think she is dating anyone now.

But then there is me. Ive been in 9 separate relationships since highschool, some lasted longer than others, but they would never go past the 4 or 5 year mark at the longest. No one in my family has said anything but its embarrassing when everyone has been with their person for so long and here I come with a new girlfriend all the time and once my family gets to know them and like them they arent in my life anymore for all different reasons. Sometimes Im the one to break up with them and sometimes they break up with me. I want to get married and have kids but Im struggling so bad to find the right one and I dont know what Im doing wrong. Im so jealous of everyone in my family with their happy life and I wonder why cant I be happy like that too?

30 years old and I just feel like a failure who is doomed to just be in and out of relationships for eternity and I feel like a disappointment to my family since they are so family oriented and how im the oldest but the most behind. Has anyone else out there been through this? If so how did you figure it out?

TL:DR - I feel like the family f*** up because everyone in my family has been able to hold down a meaningful relationship but me despite me being the oldest of all my siblings.