You poor thing - it's terribly confusing when you are already sleep deprived, hormonal and dealing with all the other changes that have occurred literally overnight. My sister bf until her boys were 22 months and 18 mths, so figured I would do the same - seemed the best thing. DS1 came along and threw all my theories out the window. For me BF was agony and after about two weeks, and him losing weight, the health visitor persuaded me to start backing up with formula. Once you start with formula, your breastmilk will start to decrease. I can remember crying hysterically the first time I had to mix the formula - felt I was feeding my DS toxic waste and my sister came out with some very insensitive and thoughtless comments too, which didn't help. I continued BF DS1 until he was 5 months, but by that point, all feeds were topped up with formula and to be honest, we were both alot happier, once I realized that formula wasn't going to kill him afterall. A happy Mum makes a happy baby ... and husband, for that matter. When DS2 came along (17 mths later), I was determined that this one would be BF longer, but that wasn't the case. BF wasn't any easier and I decided there was no point in prolonging the pain, when it was making everyone miserable. To start with, I gave him formula at bedtime (7pm) after breastfeeding and another at 10pm only, but by 3 months, when he was routined into 5 feeds a day, each time I would BF followed by a bottle - that way, I felt he was getting some of the breastmilk goodness and time to be close and filling up on formula. However, I met a mum of a 9 month old who was still BF her son and had been in a similar situation and her Health Visitor had given her the confidence and support to continue exclusively BF and wish that that had happened with me. The thing is that by the time they are 6 months, they are eating food and probably requiring only 3 feeds a day, so it's much easier.
I have to say, feeding every 20 minutes sounds rather alot, but the good thing about doing that is it means your production will increase. If you want to keep trying for a bit longer, why don't you set yourself up in your living room in front of the TV, or a good book and make it 'me' time. I used to love the excuse to chill out and watch trashy TV during the day. If you don't have Sky Plus, get it - it's wonderful to be able to BF in the middle of the night, watching some program you've prerecorded.
Sorry this is so long - when I read your message, my heart went out to you. It's such a tough decision to make, but believe me, when I line up my boys alongside other children in my NCT group who were breastfed for months, you cannot tell the difference. Do what is right for you, and remember they are your boobs and not your inlaws!