I'm going to tell my girlfriend that she needs to lose weight.
What happens if your girlfriend doesn't lose weight?
I'll end our relationship and find a new girlfriend.
You shouldn't demand that your girlfriend loses weight. You should accept her how she is and support her if she wants to lose weight.
I don't care, if she doesn't loose the weight our relationship is over.
Losing weight is very difficult and it cannot be done on command. You need to help her understand that you're concerned for her health and offer your support. A relationship is based on many things and not just appearance.