My heart goes out to you, it really does. And when I asked the question about why u would want to go be with ppl who hurt u, it was really a rhetorical one.
Yes the dynamics involved in this kind of situation is messy & full of tension.
You r well aware of the psychological & emotional components that are involved but that awareness itself is obviously not enough so maybe u need to look for ways where u don't feel so much of a victim & find ways to gain control over at least some aspects of the situation. Most of the advice given by the other posters here is excellent but of course only u can decide what will work for you. If journalling hasn't worked, try something else. Some of the things I can suggest are:
Be grounded in the moment. Having negative thoughts about something which has not happened yet (the visit is still 3 weeks away) will just make u feel worse.
Accept that u may have some pain and that u will deal with it appropriately at the time. Take some meds with u that u know are effective. Think about yr last visit . Were there things that made the pain worse or better? Avoid those situations that worsened the pain & try to create happy moments for yrself.
Maybe encourage the whole family to go out so that u r not stuck at home. 'Out' might be a more neutral place than yr in laws home where u may feel u have less control.
Maybe encourage yr husband & son to do things with yr in laws & u can plan a day for yrself without them.
Remind yrself that this visit is temporary. It'll be over after 4 days & maybe in the future u can convince yr husband that a shorter stay for u would actually be better while he & yr son can stay on & have some serious 'bonding' time with yr inlaws.
I love Walt's suggestion of having wine. I think u should have lots of it while u r there. Not so sure aboure about the porn bit though
@Walt Oleksy 
. And yes if u feel inclined to be nice to yr in laws try that too but don't force yrself to do anything u r not comfortable doing as that may only create more internal conflict.
I understand completely about making sacrifices for ppl u love but the fact that u hate doing something but feel obliged to do it because you feel that u should is perhaps a huge source of conflict for you.
In my opinion a good relationship is one where adults especially 2 ppl living together should never 'force' the other to do something uncomfortable against their wish no matter how important it is for them. But then that's my take and u may not feel the same way as I do.
good luck!
Mala