Is the questioner considered sinful for leaving the girl after their relationship developed and he felt a lack of trust towards her, and should he go back to her despite the absence of trust?

Islam has imposed strict restrictions on a man's interaction with a non-mahram woman, such as prohibiting seclusion (khalwa), touching, and looking, in order to safeguard religion and honor. Your actions with the girl exceeded these restrictions, and the intention of marriage does not permit that; rather, she remains non-mahram until the marriage contract. You are obliged to repent from this sin, and the woman has also sinned by her negligence in dealing with you as a non-mahram man. There is no impediment to marrying her if you desire to do so and her sincere repentance, righteousness, and uprightness become apparent.