Hello L_J & welcome
I am sorry to hear of your dilemma.
I don't know where you are, but you must have heard of 'Big Brother' on TV. Interestingly, here in the UK, something has been going on, in that programme, which mirrors your story, and which might help me to throw some light on this.
I am assuming that you are a teenager, or perhaps early 20s???
Young people flirt ~ and flirting can take forms which can be misunderstood.
Friendship, too, can be quite demonstratively affectionate ~ and, so, also misunderstood between boys and girls.
You say that:
'This girl told me that she liked me as a friend and a brother'
But that:
'the way she treats me is like a boyfriend'
On 'Big Brother', the men fell for the beautiful, friendly & affectionate girl, and and felt 'led on' when she then chose her ex-boyfriend over them. This happened in spite of the fact that this girl had told them that they were only friends & that she still loved her ex.
The psychologist on Big Brother explained that people pick up on more that the spoken word and that she had sent out mixed messages. The words said 'let's only be friends', but the actions ~ hugs & kisses ~ said more. Even when confronted with this, the girl felt that she had done nothing wrong because she had explained that the hugs & kisses were only friendly ones ~ and that she had said this several times, quite clearly.
Similarly this girl told you, quite clearly, that she liked you only 'as a friend'. Indeed, to be certain that you would not misunderstand, she even added that she saw you as 'a brother'.
I agree that the way she treated you may be more 'like a boyfriend' because of the hand-holding and, particilarly, saying "I love you", but you admit that 'she does it to other guys as well'.
Personally, I think that it is unfair for girls to kiss, cuddle, hold hands & say that they love you, if all they want is friendship. They must be craving affection. Yes, people will exchanges kisses on birthdays, etc, and give each other friendly hugs in greeting, but a line should be drawn.
Having said that, I am guessing that, in our youth, many of us may have given members of the opposite sex the wrong impression, just by being friendly when this was perceived as something more ~ or of getting the wrong end of the stick - thinking that someone was keen on us, when they were only being friendly.
As for your situation, she liked you; she didn't have a boyfriend; she was friendly to you ~ perhaps too friendly.
You were never her boyfriend, but she does have a boyfriend now.
From her viewpoint, there is nothing more to discuss, so she doesn't want to talk to you, if you are going to press her for answers.
I'm really sorry, but I don't think that there is any point in asking her to explain, because she already did:
'... she liked me as a friend and a brother ...'
'... she was with a guy. I asked who the guy was and she said it was her boyfriend ...'
This is how it is for some people.
Take it as a learning experience.
You will find someone, who really cares about you, and who you care about in return.
It will be an equally balanced relationship.
This one wasn't.
Take care of yourself. You will need time to come to terms with this. Good luck