I exercise / eat healthy, lose weight get really happy and the next day I overeat and don't do any exercise?

I was the same way. I it's like I would have a ton of motivation for a while, lose weight, then lose the motivation after a while and get lazy. You need to find something to movitate you when you, or inspire you. Buy an outfit that you really like in the size that you want to be (or get out an old outfit you can't fit into that you wish to wear again) then when you feel like eating junk food, or not being active, go try and put on that outfit. That usually does it for me and reaffirms my reasoning for wanting to lose weight. Although, you should be losing weight to get healthy and stay happy, which I do, but doing this usually keeps my motivation up. Also, they say it takes 21 days to create a habbit, so just try to be active and eat right for 21 days, then hopefully it will just be a normal routine for you. Good luck!