Hi Jessica,
What you are going through sounds definitely like anxiety and OCD, I've had exactly the same symptoms and compulsions when I was younger (eg. having to "confess" to my Mom) and I have been diagnosed with OCD and anxiety.
On the other hand you're quite young and it's totally normal for someone your age to doubt their sexuality (beyond OCD), most teens do, it's just not something anyone talks about because they think they're the only one feeling that way. Getting aroused by lesbian porn is A, totally normal even for straight girls and women, and B, intensified by your fears, as in you're afraid that you're going to feel this way and lo and behold, you do. If I can give you one piece of advice, it's to stay away from porn sites, not only to avoid feeling more anxious and "convinced" that you are something you're not, but also they tend to give out a somewhat skewed picture of sex altogether which can affect your self esteem.
Rest assured that it's your mind playing tricks on you, and if you really were gay, it would feel natural and good, and most importantly, you wouldn't be scared of it.
Take care, you'll get over this.
These symptoms indicate towards ocd,but you need a official diagnosis, your therapist had diagnosed you with anxiety only but i think you need to consult a psychiatrist.