I have been there and done that a few times unfortunately.
It is a tough one to get through but you can.
Firstly are your blues because of the putting weight back on or because of other things that are still stressing you? If it is still because of other things then trying to get yourself in a happier place with them will give you more motivation to lose again. You need a new perspective. Think of what little things you can do to make it a little better each day. Set yourself a little achievabble target about something in your life you want to change and at the end of each day think about the little bits of progress you have made.
I think that people try to change too much too fast - it's human nature after all! - and the only sure-fire way to change things is slowly but surely. And it's very important to congratulate yourself on small steps. If you have a day where you feel you achieved nothing, write it down (helps me!) and learn form it - what can you do tomorrow to try to do more?
If you have close friends share it with them. They will be pleased to share in your success.
I think it is really good (in a strange way!) that you are at a low point because that is when you find the motivation to get out of it. And you can. The ponds are back on and after beating yourself up about it for a while you realise that only you can do something about it again. It's all a state of mind. You have to be determined to succeed so I had to accept I'd mucked up and that it was onwards and upwards now.
So adopt a new battle cry - "I've done it before and I can do it again" and set some little goals. Even a pound off is a huge step in the right direction
I'd be happy to have a chat about it and feel free to post/ask for support. Makes a difference if there are others who understand the ups and downs.
So - here's to the future and a happier you