If your blood sugars are being a pain, then that won’t be helping. Up and down blood sugars can make us feel surprisingly bad and will make things worse.
As for friends, it’s normal for your number of friends to drop after school, especially if you don’t go to uni or college. This is no reflection on you. I think you’d find that you’re not an outlier, and that other people your age have less friends too. If you want to find more, then you need to be more social. That doesn’t necessarily mean talking. Just getting out can help, eg to an adult education class, local walking group, hobby group, volunteering (libraries are good because there’s not much talking), or make friends online.
If it’s your anxiety holding you back, then counselling is very good. You won’t be forced to talk beyond your comfort zone. Is the selective mutism recent or did you have it through childhood. It’s more unusual to have it as an adult but there’s help out there. CBT can help, as can psychological help.
At your age, it’s all about finding your place in the world. It’s natural to feel a bit unsettled and unanchored almost, as you’re in an in-between stage. There’s a place for you out there 🙂