Wow he's not being very supportive if he's looking at other women. I would have a serious talk with him about that. Its actually not hard to go from 124 to 105 (speaking from my own personal experience, I understand every body is different, especially since you just had a baby which might make it a little harder because your body is recovering) but for me personally I usually weigh anywhere between 105-115 and it fluctuates between those numbers all the time (every couple months). I don't even pay attention to it and one month I'll be 105, then 4 months later I'm 115 and vise versa. It all depends on what is stressing me out, because some months I'll eat less if I'm stressed and other months I eat more when I'm stressed. So paying attention to how much/how often you eat can help. Mindful eating, eating mostly healthy foods, drinking lots of water, etc.

Yoga, dance or walking the baby in a stroller can also be helpful.
And there are creams you can use for stretch marks, coconut oil is natural and very good for the skin.