Good to hear you are slowly feeling better. COVID and lockdown are definitely making things harder, I've found myself trying to make game nights with people I feel comfortable with and just chatting on zoom so maybe that's something you can consider if you enjoy that kind of thing.
In terms of your GF having more fun without you, maybe it's just that she has a different side that she shows her friends, and with you, she is more comfortable to recharge, be safe, be calm, etc. Idk if that makes sense but it's just a different perspective to think about.
Also if you don't mind me asking, what things do they usually talk about, and what are your interest?