Hello, Jen !
It does sound like you have many difficult issues that are complicating your life. You do the best you can. You're not Wonder Woman. there is only so much a person can handle at any given time. If your gf doesn't trust you, that's going to cause relationship problems later. There should always be trust in a relationship and if there isn't, that's no good. but that's a whole separate issue from your son.
To me, your most important relationship is with your son first. The gf is just going to have to deal with the fact that they come second. You do what you think is best for your son. that's what matters most.
As far as losing weight, if you've lost 15 pounds in a month, that's fantastic ! Don't beat yourself up because the weight isn't coming off as fast as you'd like it. slow and easy really does win this race. You didn't gain the weight overnight, it's not coming off overnight. That's why you need to be patient with yourself and your body.
Change takes time.