There is a "sticky" in the PTSD Forum that has some helpful techniques. This is part of that post.

Repeat a grounding phrase: "I'm here right now."

Give yourself permission to address one thing at a time ("Rome wasn't built in a day.")

Identify (in writing) all problems you're facing. Then divide them into two groups:

Those you have control over, and

Those you cannot control.

Concentrate on only one of those issues that can be controlled.

Decide what is important and what is not.

Keep It Simple.

Hold a safe object (smooth stone, stuffed animal, watch, ring, cup or mug, etc.).

Pray (e.g. Serenity Prayer).

Exercise.

Draw.

Find a safe person.

Listen to a tape of your therapist.

Listen to a tape of self-affirmations.

Most importantly - Identify the Trigger.