Realized I've been an ******* my entire (M26) life. How can I be better?

Title sums it up. At my age, I feel like I've hit adulthood
What really bothers me is that I made several new connections with people but they never lasted. I had one best friend growing up, others were just acquaintances. In college we drift apart, I met my girlfriend that is with me for 6 years. I consider her my love and also my only real friend in life.
Everytime I read my old messages with anyone, be it my GF, be it a reply to an email my mother sent me 8 years ago, I understand the reason why: I'm a total ******* with the ones I love, can you imagine how worse it must be with people on a daily basis ?
And this seem to be a recurring event. I've had the same email since I was 10 years old, since the old cringe days of MSN to as recent as emails from 5 years ago, I'm always an ******* in my replies - imagine it in real life ?
I was an even bigger ******* when I drank, fortunately I've been alcohol free for over 500 days
I can't help but feel that I'm like Joffrey from GOT. Is there a cure to being a ****? My family and my GF stick with me all this time because they must really love something in me, but I don't want to be a burden to them, or worse, I don't want to lose any of them and end up alone in life.
Men, did you ever realize you were not as good as thought you were ? How can I be less of an ******* to everyone ?
Edit: I wrote this in the middle of a existential late night crisis before sleeping. So many great answers here, I'm taking my time to read and reflect on them. I've also received many private messages with personal experience from others. The best part is everyone is trying their best to help, and I hope this thread may be useful for whoever feels the same way!


Source.