Sorry to hear you are so stressed out about this. It sounds as though you would be better staying there, but obviously things need to be sorted out. It also sounds as though you are well qualified and valuable to the company.
If you accepted this current role, did you have confirmation of the new job in writing and did you sign a new contract or agreement or anything? You have clearly accepted the job by way of continuing to carry on working there and doing the job you took.
So you can't really say 'can they force me to stay' as such as you have chosen to do that job in the first place (and when it suited you). I don't think your maternity leave is relevant in this (in fact its nothing to do with it really) now, particularly at this stage.
So, if you have an issue or complaint then you have to go through the proper channels and do a formal grievance. Hopefully you will get a resolution, you need to be able to say what you want out of it. Would
you be happy if she changed her ways? She may not be fully aware of how you feel.
You at least have to give the Company (and ultimately her) the opportunity to resolve things satisfactorily. If you go through the whole process and it still isn't resolved (and you have to be realistic in what you hope to get out of it/achieve) then you could possibly resign and claim constructive dismissal.
For this to stand a chance, you have to demonstrate that the working relationship had become untenable, that working there and the way you were treated was so bad that you could no longer work there, that the implied mutual trust between you and them had completely broken down. If they go through the grievance process as fairly as they can and offer options/solutions then you will have a difficult case if it is just down to you not getting on with one person.
You need to log everything I am afraid to support your case should you need it.
It's a shame that its got to this stage already but you seriously need to address the issues and start the ball rolling - you may be surprised at how well it turns out. Sadly the phrase constructive dismissal always crops up/is considered a good way out but its a minefield and difficult to prove