Well they say that it can happen any time within the first year, but personally I've known it happen even after that.
Let's be real, kids are really really really hard work. It's exhausting, emotional, and often times a thankless job, feeling like you do is so common.
Especially with the first, it's like being hit by a train. How can something so perfect and wonderful make you feel like you want to scream into the abyss and bury yourself under the duvet in silence for a week?
I can only speak from personal experience but it's worth looking at (if you can) what exactly is getting you down. Is it the lack of sleep? Feeling isolated? Do you have enough help and support? Do you miss things you can't do any more? If you can link it back to something specific after some reflection, then it's a great place to start in working out how you can get help in those areas. For example I knew my depressive episode after my first was because I was utterly wiped out. It took some effort and we pulled some strings but I managed to get a few nights away with just dh (baby with my mum) and I slept, and I ate when I wanted to and had a shower when I wanted and read a book in silence and it was glorious! But it wasn't a magic fix, I knew I needed a longer term solution and thankfully dh was supportive and it marked the start of a more even share of the work load, we worked space into the diary for me to meet friends for coffee or get a lie in and it helped hugely.
However if it's more of a general low, can't really pin point it then your gp can be a great help. I had this with my second and started taking antidepressants and it changed everything. I feel much more able to cope, the lows aren't as low and the highs feel so much more frequent and enjoyable.
I suppose what I'm saying is there is hope, there are things that can help. It all starts with a bit of reflection and asking the right people for a little help.
Really hope you find something that works for you and start feeling brighter soon xx