All you need to do is call in sick on the day you are due to return to work from your annual leave and self-certificate until you have seen your doctor next Wednesday - see here: www.google.co.uk/url?sa=t&rct=j&q=self-certificate%20uk&source=web&cd=2&ved=0CEAQFjAB&url=http%3A%2F%2Fwww.nhs.uk%2Fchq%2FPages%2F1062.aspx%3FCategoryID%3D68%26SubCategoryID%3D158&ei=KY2FT6euNuid0AWdnaS-Bw&usg=AFQjCNG-X-tzbX3opStf1vSBnjNxbykrLQ&cad=rja
As it is apparent is that you are unlikely to 'get well' in a couple of weeks, and that any benefit you gain from having a break from your present employment will be dissipated as soon as you return to work, you're best advised to use your sick leave to rest, recuperate, and give thought to handing in your notice regardless of whether you have found another job otherwise it's probable that, after a brief absence, you will simply step back onto the treadmill of hopelessness and despair.
With regard to the money you were persuaded to loan your mil and which you have now recovered, revisiting the angst you feel at being 'disrespected' and 'pushed around' can only serve to prolong your discontent.
This unfortunate experience is the type that can easily happen to any of us when we loan money to those who may not have the same sense of obligation or honour as ourselves to swiftly repay such debts. Regard it as a learning curve; you've discovered some harsh truths about your ils that you may not previously have been aware of and you now know not to put yourself in the same situation again.
If/when your mil is diagnosed or a SS care plan is put in place for her, she will be entitled to claim the new equivalent of attendance/carer's allowance and it may be to your advantage, and that of your household, if you research what additional benefits are available both for your mil and also possibly for your ds which will enable you to give up work, or find local part-time employment that may be out of your chosen field until you have completed your further studies - to this end I would suggest you regard this time as a 'sabbatical' rather than an exercise in giving up your ambitions for the future or martyring yourself to the cause 'because no-one else is prepared to do it'.
With a severely disabled ds and a mentally impaired mil to care for, your primary objective should be to find practical ways of relieving as much pressure as possible from yourself and your dh so that you can spend quality time with ds and each other, and thereby rediscover some of the joy in life.
Of course you are not an intrinsically 'inferior' being. However, it seems to me that your misguided but seemingly deep-rooted belief that you are not 'good enough' has coloured, and is colouring, your view of your world and is painting it much darker than it is in reality.
Without knowing considerably more about your history and to what extent the birth of your ds and his disabilities have impacted on your belief system, it is not possible to recommend a psychotherapy that will address a condition that would seem to be far more than the type of 'depression' that can be alleviated by ADs.
It could be that a short course of cbt will have the effect of putting a plaster over the wound, so to speak, but in order to fully 'heal' it may be that you will need to embark on long-term regular sessions of a talking therapy in order to explore, modify, and alter the erroneous beliefs that are sustaining and fuelling the anger and resentment that is blighting your life.
Ask your gp to refer you for assessment to determine which therapy will be of most benefit to you and, in the meantime, explore the alternative stess relieving therapies such as meditation, hypnotherapy, EFT, etc, which I believe have been suggested to you in this and/or your other threads.
As springy has said, mental health provision in the NHS is sadly lacking but even if it were funded to the full, it would still be down to you - as it is to all of us - to put in the time and effort it will take to bring about a positive change in your thinking and outlook.
In short there is no magic wand and, within the boundaries of certain unalterable immutables, life is what we each make of it. I hope that you will take the necessary steps to make yours more rewarding and fulfilling than it has been since you first posted here.