Welcome to this space, I am so pleased that you have found yourself here to share how you are feeling and to get some support. Being in a relationship is alot of work when things are going well, to maintain and to ensure each person gets their needs filled and that things go smoothly, however when things are not going smoothly that is even harder..and when essentially the issue is not between you are her at all. This must be so very frustrating and I am so sorry this has lead you to feel like you have no choice but to leave this life.
Can I tell you that you are not weak, and your feelings and how you live in the house and in the relationship is very important. This is a tough time.
I can give you some tips on what I believe may help the situation, and firstly I think what I would do is write myself a list, of all the things that I find annoying and all the things I don't like. I would then put a solution next to that point, if you can think of one. See I feel like when having conversations like this is more helpful to have some solutions rather than dumping a whole heap of "problems" on people.
I think a conversation is required here to let your partner know how badly this living arrangement is impacting you. I am sure it is impacting her too in that she feels so torn between her partner and her children. If you can share with her some of your concerns and then also show her some of your suggestions for a solution that may really help, and help her too. Saying to "just turn off" does not work and is not helpful so you also need to mention that to her too. Also, these are not small children and should be able to consider the other people living in the house, especially if they are not contributing and living there as dependents.
This is your home and you have the RIGHT to be happy there.
I think also making it clear it is not a choice for her in that she has to choose you over them, it is a discussion as to how you all live together harmoniously.
I hope that you are safe blade.spg, but if you are not please call 000 and have an ambulance come to you. I am just so proud you have reached out here today and to let us know how you are feeling.
Also I just wanted to say that it is actually ok if you don't get along, no one is forcing a relationship with you and the kids, but there must be respect and peace and you all must be able to live together considering other people in the house.
I hope to chat some more to you.