Wow, sounds like a really difficult time. I'm kinda going through it too except I only have one child-- a 7 month old very active little boy! But heck, now I feel bad for complaining because you really have it tough. I agree that you should no longer vent to your friend. It sounds like she is very wrapped up in the illness of her child (as she should be) to understand what is happening to you. I think for your mental state of mind don't vent to her anymore. Invalidated feelings are the worst when all you need is someone to just emotionally hold you-- I know about this b/c DH is like that : (.
Try to go to LLL meetings. Also look up for new mom groups in the hospitals in your area. As a new mom myself I looked for these and started going just so I have other moms to talk to and possibly get some advice. There are always children running around. I'm not sure how difficult it would be with your pain but please try to go. It has been a saving grace for me because I look forward to them and I feel like I can relate to some of the moms.
As far as going out-- can you go out after your DH comes home from work? Even if its once a week for about an hour or two? I do that sometimes although not nearly enough.
The last thing I want to say is to hang in there and take it day by day (such a cliche huh?). Your words ring so familiar to me and I want to just call you so we can both vent or heck, have a margarita or two. Sometimes I just cry-- and then I feel better. I hate crying but there is nothing else for me to do sometimes. Other times I just let the house be and not do anything but play with my son. And if my DH doesn't like it well, I've learned not to let it bother me. He really does think his job and mine (being a SAHM) are comparable. I've quit venting to him (which, sadly, is another post all on its own). Lots of hugs and well wishes to you. Hang in there.