@TomTomBinks,
Thanks everyone for the comments.
I think there are several reasons.
First, when our relationship first started, we had a really big fight and he ended it, but he insisted to know one last thing, he wanted to know with how many guys I slept. Well, I wasnt completely honest about it and said a number close to the real number, but less (-5). I thought what does it matter it is over anyway.
But after it came all good and I told him that I lied about the number. Well, that is the root of it all. Because I lied about that, he has a hard time trusting me.
On top of that, I am a person that can change her mind. About social media, or food, or clothes. In my mind it makes perfectly sense, but I know that for him it is very frustrating sometimes.
Combined that is the reason why he has troubles trusting me.
I asked him why are you so insecure about our relationship? He said, it is because of you, you lied to me and on top of that you change your mind about things. also he says that whenever we have an argument I make up little lies to sound better. (while I just try to explain why I think like I think)
Now I am writing this I see that it is me that makes him insecure. But I am just not a very good person to have an argument with. I change direction very easily (also to just stop the argument), and that makes him even more angry.
It is hard to be yourself, and also pay attention to every action you do. It doesnt make sense, you should be yourself with your partner, but you should also take into account how he/she feels right?