Writer, I see what you mean by my question being too general. I suppose I am just beginning to realise that the way I was brought up didn't allow for me to set boundaries so it is something I never learnt to do in a natural way.
Whereas most children and parent relationships are at least ideally such that you have a dynamic where parents set boundaries with children and vice versa, so you learn through practice and their example with other people.
I feel as though I need to learn how to stop people walking over me and to learn to tell them such without resorting to becoming uncomfortable and angry/upset, and as you said taking other people's feelings into account. I guess learning to say no comfortably to 'authority' figures (such as ILs, parents etc) too.
I suppose a real life example, my partner has been diagnosed with severe depression and as such some (many) of his depressive behaviours I would find wholly unacceptable in a 'normal' person, but I am taking into account his illness, his willingness to try and recover through medication and therapy, and find that it is therefore difficult to set boundaries.
Another example is that my ILs when they are in contact are very prone to setting guilt trips and I need to learn to say no (in a nice way?) e.g. my MIL bursting into tears infront of my (very confused) children because something doesn't go entirely her way (like the ash cloud last year preventing her taking her second trip to see us in a fortnight, so laying a guilt trip on us because she was frustrated).
Another example, my mother telling me she would no longer dial my phone number because I am always busy and then complaining that we never talk. Or her complaining that I am too secretive with my life and need to share every sordid detail.
These are examples of situations I find hard to deal with therefore was thinking I need to learn to set boundaries when they arise. Does that make more sense?