OP, I have been where you are. I don't know if I have any solutions, but what I would say is that feeling flat is an entirely normal response to all that has happened to you, the end of what you must have hoped would be a long relationship, the death of your daughter's dad, and now being a single (lone) parent, which is a hard road to tread. It is completely understandable you aren't bounding around right now feeling great.
I suspect there isn't a quick fix, as in a supplement you can take, or a social event you can attend, but bit by bit, you will keep going through this stage til you start to see a future again. I think the hardest thing when you are in this situation is to lose that future you think about, and it's that loss which is why everything seems so empty. Be very kind to yourself, keep going with the exercise and activities, and dig in to spending time with your daughter just in a consistent way, she won't be around nearly so much in a few years so I bet this time will bond you a lot, although teenagers are hard, it will set your connection up for the future. Your dog will get you out and about. Over time, you'll find some new things to do, more freedom as she gets older to leave at homoe on her own, start making plans, perhaps change the house you are in right now a little whilst you are waiting to move away, and eventually get yourself back on your feet.
It's too soon to give you advice on dating, but there's some great threads on that if and when you are interested, or if you are not, then there's a lot of peace to be had with your daughter and your dog.
I'm sorry I don't have any fixes for this, but you will keep going during this time, do the things that you do that make you feel better, and find a path through.