I have just had my second baby, and she is 4 we...

I have just had my second baby, and she is 4 weeks old. I'm breastfeeding her, but am finding at certain times of the day I just can't satisfy her. It always seems to be in the afternoon/evening and late at night. I'll have her on the breast for an hour swapping sides, winding and changing her nappy and she still seems to want more and would drink approx 50mls of a bottle. I would like to keep breastfeeding her, but am finding it a bit hard. The middle of the night feed is the worst where she wakes at approx 3 - 4am, feeds for an hour and then can take up to another 30mins to settle. This would be fine is she was my only child, but my DS is 23months old and is up at around 7.30/8am. Part of me feels like it would be easier to put her on the bottle, but I also don't feel that it is a good enough reason for me. I have milk there, I wake up in the morning and my breasts feel like melons. But I don't know what to do to make it any easier. At the moment I am giving myself little milestones like I'll make it to the 6 week mark, then 2 months and then month by month until at least 6 months. But at the same time I don't want to put myself through hell either. I suffered from PND after my DS and don't want to go through that again. I really love breastfeeding, I love the fact that I am making her grow and develop and I love just looking at her when she is feeding. So much so I had to go to the physio to fix my neck. I'd really appreciate any advice anyone can give me. At the moment, I feel like I'm damned if I do and damned if I don't. One way or another I'll end up punishing myself. Kelly