kafka sounds as though you have been having a really tough time. at the lack of support for you at work.
You are right to think that an ET claim would be incredibly stressful - as you have found just dealing with your situation and putting in a grievance it can really take it's toll and it is not uncommon at all for individuals to be off work with stress when going through something like this.
Obviously without more information it is impossible to say how good a claim you would have - you could have a case for sex discrimination depending on what exactly has happened surrounding your return from maternity leave, and there could be a case for constructive dismissal if your position has become what is called untenable, if your working situation is so unbearable you literally have no option but to walk. However, constructive dismissal is notoriously difficult to prove and claim, and is very stressful.
It is a very mature (if I can say so without sounding patronising) and self-aware attitude to realise that the 'right' thing is not necessarily the best thing for you, and sometimes you are right, it is best to walk away for your own good, for your health and to enable you to get over this quicker and move on. It does seem 'wrong' to think this and to advise this - the natural instinct if someone has been treated badly at work is to seek 'justice' and redress and is of course what you are entitled to do.
However many people embark on grievances and tribunal claims etc in a bid to prove that they were treated badly, and to get compensation, but go through the mill in doing so and come out perhaps with some money but having been through months/years of a stressful process and not having started the process of 'moving on'.
It is not humiliating to walk away if that's what you choose to do, it is taking the mature and informed decision that that is what is best for you and your health and what will enable you to move on quicker in the long run.
Of course your employers deserve to have to fight a tribunal claim and deserve to be held to account for what has happened to you, and if you choose to do that, you are absolutely within your rights and it may be the right thing to do. But I would always advise anyone to take what is the best decision for them and in some cases the best thing is to walk away, however hard it is to do.
Only you will know what is the best thing for you. It may well be worth doing as Twig suggests and talking to a solicitor, giving more detail than you are able to here, he/she will be able to talk you through exactly how good a case you may have, and you can ask them to be as honest as possible about the costs, both financial and emotional, and about realistic timescales. You may find when you have more information that you change your mind and decide to take this on.
It is really important that employers are held to account when they break the law, and when they treat employees badly, so it is important that some people do take these things 'all the way'. But if your health has already suffered, you are feeling 'out of fight' and just want to move on, it may well be that you shouldn't be one of those people. You have to think about yourself first and foremost, not about the 'greater good' of holding employers to account.
Think about talking to a solicitor about the case you may have, you may change your mind and decide to bring a claim, in which case there will be plenty of support and advice here for that as well. Whatever you decide to do, I hope that you can recover quickly and start feeling more positive about your future.