Hang in there!!!
My little boy is just gone 5 months and the similarities between him and Izzy are incredible. From about 4 months he just changed his routine totally - sleeping, feeding, feeding to sleep. I felt like i'd regressed totally with hime. I didn't bother too much with trying to rectify it until a few days ago (mainly because we've been really busy!) and as soon as he hit 5 months he's just "changed" again. Sleeping through again, settles himself (which he's never been brilliant at doing) and his daytime sleeps have gone back to normal. He still fluctuates a bit and we're not on a strict routine, but his tired signs seem heaps more obvious now. It's like they've reappeared, which makes it much easier for me to follow iykwim. So my advice would be try and hang in there and just do whatever gets you through...probably not much help, but I find its a little more comforting if you know others are going through it.