Charlotte - this was me too, those early days can be hellish though!
With ds1 I didn't get past 10 days and it ripped my heart apart and I do think it was a large part with my PND at the time as I feel I had failed.
ds2 I was determined to see if I could get further as since ds1 I realised the support I had got from 'those in the know' was dire, really dire, and I got a lot of support beforehand from MN in relation to bf and c sections. Baby arrived and the support in the hospital and even CMWives was FANTASTIC, complete opposite of the time with ds1, however it was MNetters I swear who got me through 8 weeks of bfeeding until the time I could get to RL bf support group. However as sometimes can be the case, ds2 and I got through rounds of issues, eg mastitus, soreness, over supply, silly moo who tried to put me off in public, just to name a few things, then finally I found myself going around in circles with emotions and ended up seeing the Dr bawling my eyes out as I felt so crap. He listened and was convinced I didn't have PND (at 3 mths or so) but was over anxious about the bfeeding. I was begining to lose count of the times I bawled away in bf support group and had things like a good latch confirmed etc, but was still in pain, so before I made a decision I went back to exclusively bf for some weeks (had a spell of mixed feeding) and found myself still on the merry go round, so decided to go ff, but expressing until I stopped producing larger amounts of milk, which made a bottle of ebm a day for several weeks. Once this decision had been made I felt A LOT better, pangs of guilt yes, but a lot more sane and decided to concentrate on what I had achieved rather than think I was a complete failure or selfish for not continuing. I know in my heart I did what I could, (think I made the bf lady nervous though when I asked her about pnd medication because I noticed a change in response around me) Didn't need any medication in the end as the Dr had been very observant I think in my appt.
Whether it be a day, weeks, months I think there are some of us where it doesn't go quite to plan despite our best intentions and as already mentioned you gave it a go. I think when a decision is made, it can be about anything not just feeding then one always feels some relief and better about things.