Overthinking everything / anxiety

Hi kind strangers, my name is Oliver and this is my first time writing on this subreddit. To put you in context, I am 18 years old and in a relationship that is 2 years old. My girlfriend and I love each other like crazy but, like 8 months ago I've start doing some extreme anxiety about her. I trust her so much to the point that I don't care if she go out in a bar, if she talk to other guys in a bar ect because, after 2 years with her, I know her pretty well and she wouldn't do something that could ruin our relationship. The problem is that I overthink litteraly everything at the point of making me having panic attack. For an exemple, yesterday, she woke up in a bad mood and told me she needed her bubble and I've litteraly panic thinking that she didn't love me anymore or things like that. I am seing a psy and it is going well when I come out of her office but when stress is high, I broke and just start panicking on everything. I don't know what to do, sometime I control myself, sometime I just broke and this is turning toxic for both of us. I really need help on this because I am starting to have some black ideas in my head and know that if I continue being like that, I will loose her and that's for sure, no one wants to be stuck in an arguments each week because the other isn't okay mentally, even tho she love me as hell. Thank you for your comments in advance!