@eskie aRecently I was lucky enough to have a 7-day meditation and mindfulness course here in my country (I'm Buddhist so this is quite normal for people here for the temple stay to do meditation and actually I did it before but never really understand the core or it doesnt work like this time ) I'm going to write a review about it soon since I think it may help some people like me such as you. But here are some quick tips for you to help you passing these bad days...
First, I'd like you to start practice mindfulness anytime anywhere as long as you remember. I want you to focus on your body movement like while you are lying to sleep you just think that I'm sleeping, or while you're walking, seeing TV or browsing the Internet, you just see yourself as another person and think I'm walking, watching TV, etc. You may think this is silly but it's a good start as I tried it before and it works! (But you just dont think that it will work in a couple of days but keep doing and be patient for amazing result!). Then you just become more aware of your thoughts like when you think I'm such a dumb or shy or whatever your mind tells you that you are, you just think that ok I'm thinking this and let go. Do this as often as you can. It sounds strange and you may wonder how it could help but I would say this is the first step because when you aware more of your thoughts, you will have more consciousness and at the next level when you are expertise at this, you will understand that it's the old unhelpful pattern that made you stuck like a vicious circle that get you out of nowhere and you can really let it go. It's really important that you just acknowledge them as they are and let go. If it creates more bad thoughts, then you just acknowledge the new thought. It's hard for me at first coz I think I was such a bad person by thinking so negatively or criticizing people or myself so much but after you practice this for a while, you can realize that most of them are not true and not believing it anymore. Then, you will be free from your own thoughts (which I wanna remind you that your thoughts are not yours. It's just the thoughts which belong to no one and once you can detach it from yourself, you will be free as I do and realized that what they are telling you are mostly not true.)
I wish you best of lucks practicing this and you can drop me anything if you need my further suggestions.