Hi there hellocupcake, and welcome to Scarleteen!
What you're describing is actually pretty common! There is definitely a difference between thinking about/desiring sex and being physically aroused - otherwise, like you said, a lot of young people would be walking around hot and bothered all day. The person you were talking to might use that definition to describe "being horny", but for most people, there are more than a few things that need to happen in the process of thinking about sex, feeling aroused, and feeling relaxed and lubricated enough to have any kind of intercourse.
You might find reading
With Pleasure: A View of Whole Sexual Anatomy For Every Body really helpful, as it talks about how many factors actually go into that process.
As for your question about ptsd and anxiety, they can play a big role in how someone experiences arousal or very little. It depends on what types of things prompt anxiety or trauma-induced emotional responses, and whether those come up in thinking about or engaging in sexual activities. If you read the piece, you'll see that a whole lot of a person's experience of sexual activity is grounded in how a person is feeling mentally and emotionally, so folks who have a history of emotional distress or trauma (especially if it had to do with sexual or bodily violation) might have more complex patterns of desire and arousal than someone who doesn't.
Do you feel like you have access to the right resources and support about your ptsd and anxiety?