Better to say "no" to DH than to say "no" to DS!!!
Firstly, sex is not vital for a relationship to function, it's nice, yes, but not vital.
Secondly, I don't think bf itself is responsible for loss of libido. Sleep deprivation is far from an aphrodisiac, and the kind of long term exhaustion that comes from 19 months of intense parenting is not going to go away over night. There is some evidence that shows that bf can impact on your hormone levels, some women do say they need lubricant when they are bf, but none of it is proven, and tiredness can impact on this response too.
Finally, I also "suffered" from a Boob-identity crisis after I'd had DD1. I have always had big boobs, and they had always been pretty central to my sexual self-image. Suddenly my boobs were a feeding tool and I was a bit unsure about it all. I didn't like them being fondled, as someone else has said, by the end of the day my boobs are a bit touched out, so the last thing I want is someone else tweaking my nipples!! The thing that has worked for me is foxy undies. These were a challenge in it's self as finding 32F sexy bras has not always been easy, but now you can get some nice stuff from La Senza and Ultimo amongst others. Put on a nice bra, and keep it on. Your DH can still enjoy your cleavage, but you won't be swatting his hands off your nipples, and you'll feel sexy, because your boobs aren't bouncing about leaking!
But don't feel you have got to be having lots of sex, and don't blame your lack of libido on two breastfeeds a day! Your DH needs to try to understand that you are tired, it's a basic fact, not an excuse, and he needs to show his love and support for you by not pressuring you into getting jiggy. Have a few lie-ins, a nice soak in the bath, some time to yourself, if you feel rested and unpressured you are more likely to feel amorous, even if you have to psych yourself up to it to get started, you'll find you do get into it.
And by pressure I don't just mean pressure from DH, I also mean the pressure that comes from being on-call 24 hours a day, 7 days a week, 52 weeks a year. Being the centre of the universe for a toddler is very hard work, be you SAHM or WOHM, you are still a mum every second of every minute of every day, and this can be very draining. I found once I had a couple of chats with DP and explained that sometimes I felt like I was spending my whole life meeting everyones needs he managed to see it a bit clearer from my point of view. And once the pressure was eased I felt more inclined.
Being a mother changes you in so many ways, physically, emotionally, hormonally. I think we are misled by "society" into thinking that we will pop out our babies and within 6 weeks be breastfeeding, working, cooking gourmet meals and ravishing our DP/H's like wanton teenagers again, and the reality is very different!
But 2 feeds a day are highly unlikely to be having that much on an impact on your hormones, so may be look at ways to help you get some "you time" so you can rest up and relax, share the parenting a bit more, that kind of thing.
Good luck!