Here are some small tips to help you cope if you are feeling overwhelmed in the moment. They are not a replacement for support or medical assistance, and cannot take into account your personal health or circumstances. If it doesn't feel ok to try them, then don't! If you start trying them and it doesn't feel good, then stop. You know best what is right for you at a particular time. Some of them may be very familiar to you, and sound useless or obvious, so I've tried to explain why they can be helpful.
Notice your breathing. Try blowing out slowly, as though you're blowing through a straw (this helps activate your diaphragm to help you blow out), until you've blown out all you can, and then just let the in-breath come on its own. Do this for a while until you feel your breathing settle and deepen. Often, the advice to 'take a deep breath' is hard to follow because our lungs are already full, and we're just breathing with the top part of them. If you focus on blowing out, that can be an easier way to regulate your breathing.
Keep your eyes open, and look all around you. If there is no immediate danger (and if there is, get out of it as soon as possible!), then tell yourself what you can see - the more mundane the better. A green carpet, a light switch, a light, a picture of a tree, a window, etc etc. See how much detail you can use to describe the things you see. If this is really difficult, try just counting things. Keep doing this until you feel a little calmer. Using your 'thinking brain' in this way can keep you present when you're feeling overwhelmed or spacy, and help you calm down.
Move around. For many of us, when we start to feel very stressed or overwhelmed, we can begin to go into 'freeze', or become very still - often we don't even notice we're doing it. If you think you might be doing this, try doing just tiny movements to start with, like blinking, and wiggling your fingers and toes. If this feels all right, then try larger movements - your hands and feet, arms and legs. Try walking around a little bit. This helps to come out of 'freeze', which can feel powerless. It might be helpful to make some sounds, too.
Take some 'Rescue Remedy'. This is a calming mixture of Bach Flower Remedies that can help us calm down just enough to get through a short stressful period. It is available as drops, spray or pastilles, and you can get it at most chemists and health food shops. Rescue Remedy is very gentle indeed, and perfectly safe for most people to take, but please check the box and information to make sure it's ok for you.
If you know that your stress is coming from memories of the past, remind yourself when and where you are. The date, the year, your age, your height, anything about the present that is different from the past. What can you do now that you weren't able to do before? Who do you know (and like) that you didn't know then?
Access your resources. What are the things in life that help lift you, even a little? Some small things might be:
Creative activities like cooking, drawing, writing, singing
A nice smell - smells can be especially good for dealing with stress, as they have a more direct access route to areas of the brain
Something pleasant to touch or hold - a smooth stone, a soft cushion or blanket, a furry pet