Why is my gf [22F] not supporting me [26F] joining a weight loss group?

Why is my gf [22F] not supportive about me [26F] joining a group to lose weight?

I really need to lose weight, it’s not like I only need to lose a few pounds it’s literally another person. My life’s took a standstill for a couple of years now and I have put loads of weight on. I feel like this would be the starting point to change my life around. I’ve always struggled with my weight, back in 2016 I lost of a ton of weight but unfortunately I stopped going to this weight loss group and over the years I’ve put every pound back on and more ? it’s honestly gutting but I really need to try and get back to where I was then. I told her I wanted to re join this group (it’s Slimming World by the way) and she was instantly negative about it. I told her I could walk there for exercise as it’s a 26 minute walk and told me “ In reality you wouldn’t make it, you’d be f****ed and be too sweaty and it would take you way longer than 26 minutes” I told her yeah it would be a challenge for me at first but I’m not incapable I’ve been on longer walks and then she kept questioning me why I need to go to a group to do it, I explained that the support really helps me and she just was laughing at me saying the only reason it works is because of the fear of them weighing you not because it’s a good diet and said you know my opinions of Slimming World.. baring in mind she’s very overweight too and wants to lose weight also I just felt like she was being insensitive and unsupportive, it’s annoying because I support her intentions of losing weight by calorie counting, I personally struggle that way but everyone loses weight differently. I just feel really irritated and feel like she just will think I’ll fail.. like she doesn’t believe I can do it. I don’t know how to resolve this if that’s how she feels?

TDLR; my girlfriend [22F] isn’t supporting me [26F] on the decision of rejoining a group to lose weight, why is is and how do I go about tackling this issue because it’s hurt me and makes me feel like she doesn’t care that I want to better myself