LittleD0t I switched with DS1 when he was a month old and I'm heading that way now with DS2. The problems have been very similar ie poor weight gain (DS2 is still not back to birth weight and only gained one ounce in a week despite feeding all day long), what seems like ineffective feeding as DS2 literally wants to be on my boob all day and I'm barely having time to shower, eat a meal etc as he'll feed then cry for a feed 10 minutes later.
I'm seeing a BF clinic today as really I would like to just know what the problem is. I'm never getting much when expressing, so I don't know if it's supply or what.
When it comes to stopping, it's recommended to replace one breast feed with a formula feed slowly. Only express to relieve any discomfort/blocked ducts, not to replace that feed. With DS1 we introduced one bottle a day at 2 weeks, a second at 3 weeks, and then an extra every couple of days. This is the best way to do it to prevent mastitis. If you get any blocked ducts just express through them, you could freeze the milk then if you want.
You can stop cold turkey and express to relieve discomfort, as I know people who have done this.
I'm sad too that it hasn't worked for us, but I remember what a relief it was when I stopped with DS1 and it made us both so much happier. I'm going to keep going until Sunday for sure as DS2 turns one month old then (I want to do the same as I did for DS1) and then see how I go. I would love to be able to stick with combi feeding but I don't know if it'll work as DS2 just never seems satisfied, even for more than 10 minutes.