It's a teenage crush, and that kind of obsession is normal when everything is The End of the World (cf. Romeo and Juliet, every episode of This So-Called Life). What isn't normal is the lack of respect for boundaries, which maybe haven't been clearly asserted.

Between the ages of about 12 and 25, the brain is changing and there comes a phase when teens are in a sort of 'bubble', where they feel completely disconnected from others. This can feel strange and terrifying, leading to emotional outbursts. It also makes it very difficult to pick up on social cues and behave appropriately. It's a natural part of human development, it's a phase, and it passes.

The crush will soon fade, but in the meantime, those boundaries have to be firmly established ("it makes me feel really uncomfortable when you text me to say that you love me, because it feels like you are trying to guilt-trip me into saying it back") - be clear and specific.

A little bit of distance, for now, would be best. If the friendship has value, it can be resumed when all this is in the past.