Vegeterians, and vegans are very often deficient in B group vitamins, particularly "vitamin B12". Enter it in the searchbar at www.mercola.com and view the articles; I get 6 monthly injections; you may prefer the sub-lingual spray.
Vitamin, and mineral deficiencies adversely affect mental health, and can cause, or exacerbate many serious disorders, such as depression, schizophrenia, and anxiety. Have blood tests done; you are very probably deficient, and quite possibly, seriously so.
Females may benefit by balancing the DHA, & EPA versions of Omega 3 fatty acid in krill oil with ALA flaxseed oil Omega 3, or at least one heaped tablespoonful of ground flaxseed, daily. Vegetarians & vegans: Google: "Omega 3; algae; supplies" and also use ground flaxseed, or flaxseed oil, daily.
It's important to regularly monitor, and deal with a negative internal monologue (self talk), or mental process, such as disturbing thoughts, images, impulses, or emotions, by the process of (a): recognising it, and (b): challenging it immediately. Technique For Re-Programming Negative Thoughts: When you notice something negative, such as: "I can't do this/ am never going to get over this!" or: "Why am I always so useless/such a loser?" or even an image, emotion, or a memory; recognise that it is being generated from the negative part of your mind.
After identifying, and labelling it, visualise a large, red, flashing, "STOP!" sign, and/or possibly a stern faced person wagging an index finger at you in a negative manner, then say to yourself as forcefully as you can, even aloud in a big voice, if alone: "I know this tactic: GO AWAY FOR A WHILE !!!" You may want to use either: "ruse", "ploy", "game", or "trick". In the case of an image, visualise a large "STOP" sign, or your preferred version. Some people go so far as to keep a wide rubber band in their pocket, then put it around their wrist, when they catch themselves backsliding, stretch and release it, as a method of reprogramming their mind sooner, but I don't regard it as being strictly necessary. Remember to remove it, afterwards, if you use this method. Try replacing a negative thought with a positive affirmation of your choice, like: "I am a unique individual, with my own set of skills, and good points", or "I may not be perfect, but I'm doing the best I can, right now".
Practice a relaxation method, daily, and when needed, such as http://www.drcoxconsulting.com/managing-stress.html or http://altmedicine.about.com/cs/mindbody/a/Meditation.htm
or http://www.wikihow.com/Meditate and/or Tai Chi, Qi Gong, or yoga. Imagine, as vividly as you possibly can, that your concerns, anxieties, worries, fears, your self-recriminations, all the agonizing "I'm-so-Stupids"; every painful "I-can't-do-it", etc., are huge raging, hurtful bulls snorting and charging at you. You see them coming at you and you hold out your red matador's cape at arms length and simply let them charge right by. Every time they come at you, hold out your arm with the red cape and let them rage and snort and go right on past. A free E course in CBT is at: http://ecouch.anu.edu.au/welcome Most people are suggestible, to some extent, so consider professional hypnotherapy, or hypnosisdownloads.com has one about stopping negative thoughts.