1. Just relax.
This is probably one of the worst pieces of advice anyone can give. It's up there with "Take a deep breath." More oxygen won't solve my problems at the moment! If someone is feeling anxious about something, telling him/her to relax is not going to magically erase his/her stress. On the contrary, having been told this personally, I get even more riled up. I start going on a tangent about what the issue is and why relaxing is not going happen. I feel the need to justify my anxiety even more when someone tells me to relax.
2. Let it go.
I wish it was this easy! Dwelling on something is very common for people who have anxiety. It is hard to want to confide in someone when the advice he/she gives is to just let whatever is bothering you go. Maybe through talking about whatever it is that is causing you stress, you will be able to work on a solution. When someone tells me to "let it go," I immediately want to either reexplain why I cannot let it go or just stop talking all together because I feel as if that person does not want to hear more. This can cause you to bottle up your feelings, which is not healthy whatsoever.
3. You're overreacting.
In certain situations, being told you're overreacting is ok. If I directly ask someone if they think I am, it is one thing. That shows I am realizing that it is possible I am. But if I am really distraught about something and someone approaches me saying that I am overreacting, that is very agitating. Nobody should ever have their feelings degraded or softened. The most important thing anyone can do is just listen to why the person is anxious and stressed and then give logical advice about it. The person listening can even share his/her feelings on what he/she would do. Personally, if someone directly says that I am overreacting about something, it makes me feel more anxious and does not help walk me through my emotions.
4. It's really not a big deal.
Clearly this is a big deal. Maybe to other people it's not, but clearly to the person who is struggling, it is. This is another statement that can make someone feel very insecure about the way they are handling themself. Anxiety is something that is hard to control. Even if something doesn't seem to be a huge deal to an outsider, the person experiencing this stress needs to be comforted and effectively talked to in a manner that does not minimize it, but rather addresses how it can be solved.
5. Don't think about it.
If someone tells me not to think about something, great. I am so glad you think that is possible for me! Unfortunately, the likelihood of me truly blocking out a problem completely is slim to none. This is not an effective approach, in my opinion. Even if you are able to temporarily distract yourself, eventually the stress over something that was not previously addressed will resurface. Also, realistically, how many people with anxiety can just stop thinking about what is bothering them or what is making them anxious? I think people SHOULD think about it. Think about it in a productive way. Talk it out, write down your feelings, make a pros and cons list if that is applicable and tend to the stressor instead of ignore it.
Hopefully you can use what not to say to guide you to understand a more appropriate way to console someone who has anxiety. The most important takeaway is that suppressing someones feelings is never the answer. ALWAYS make sure he/she feel heard.