I am currently cheating on my girlfriend.
What is your reason for wanting to cheat on her?
I heard that in long relationships people usually start to develop in a similar way and that we might get bored with eachother sexually
It's wrong to betray someone in a relationship.  Your girlfriend loves you and trusts you to be faithful to her, and it would be devastating for her to learn that you have given up on your relationship.  Developing together is a good thing, even if you no longer feel sexual passion, and you should welcome it.
So I'm like a total weirdo and a loser. Your advice is really bad.
If you feel the urge to cheat in a relationship, then you are probably better off just ending the relationship. There are times when you may have the urge to sleep with someone else, however just know that this is natural. If you truly do not care about how your girlfriend would feel if you cheated on her, this shows that you probably should not be with her.