How do I stop worring about her cheating (long distance relationship )?

We met three months ago on vacation and turns out we were living in their neighboring countries, and therefore we kept contact and became official and ever since we had a huge fight she has been distant. And even looking a bit sad or uninterested. We still talk every day for hours. We still laugh, but I feel like she’s not happy to see me as she was before the argument was about Her not telling the truth that she went out with another guy after I left vacation. but she actually told me that she didn’t tell me because she was forced into having sex, and still has flashbacks and told me she is feeling a bit depressed. Ever since I have been struggling to trust her fully. It doesn’t bother me because we were not officially together when this happened. The only thing that bothers me that she wasn’t honest, but I do understand why and I would probably do the same and her situation but I can’t help that I’m feeling very, very anxious all the time, and don’t know what to do. i’ve been feeling very anxious and weak and insecure and she told me that I would conversations haven’t been very fun because we only talk about sad stuff which I do understand but she kind of put me in that mood too, but I’ve been trying to make sure she can be there and talk to me and I will listen if she needs it. And I try to bring her mood up, but I don’t know what’s up with her. She told me it will be like this for a while, but she is there and reassure me that she’s my only one and she misses me and that she wants to be with me for the rest of our life, she tells me stuff like that and I do feel reassured, but after the anxiety comes back and I start to think that she might be cheating on me, especially now she told me she’s putting in a rod in her arm that is going to make her not able to get pregnant for three years And we are in the longest relationship so I don’t see the point of that if we don’t see each other very often and I don’t know I have trust issues.
How do I stop worring about her cheating (long distance relationship )?
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