The food was so good I couldn't help eating too much.

Literal

Cooking [subject-が] was-delicious-and unintentionally ate-too-much.

The て-form of おいしい (おいしくて) links it as the cause for the next clause. つい (unintentionally, couldn't help it) is an adverb expressing lack of self-control — the speaker ate too much despite themselves. ~すぎる on 食べる gives 食べ過ぎた. Ruefully relatable.