That’s a great question, and you’re definitely not alone in this feeling. Becoming involved in a romantic relationship opens us up to being more vulnerable with both our significant others and ourselves, so its natural for feelings of anxiety to fly around. The best advice I can give is: stay secure in yourself! Continue to perform your personal best in work/school/extracurricular activities and retain a source of independence outside the relationship, that way you feel more secure in your daily life and can bring this feeling to the table in your relationship.
I also always recommend talking to your partner — in a healthy and inviting way — whenever you have feelings of anxiety, mistrust, doubt, etc. surrounding the relationship. Start the conversation by letting your partner know you’re feeling a certain way, and that you’d like to talk through your feelings and come to a conclusion together rather than dealing with them yourself. Instead of outright saying something like, “you’re making me mistrust you,” “you’re giving me doubts” — frame it in a more inviting way by saying something like “I’m starting to feel distrustful of you because of _____.” Walk them through your feelings and frame the conversation in a way where it can be a two-way street, and I assure you both you and your partner will feel better after talking things through.