Congratulations on the progress you have made in the gym. That is a very worthwhile achievement.  It's easy to fall into the "I'll be forever alone because of my stutter" kind of thought. But you'll be doing yourself a disservice. I recommend that just like you are putting effort into the gym, put effort into some social activity. If it is not easy to find a social activity, maybe start with something like a group session at the gym, or find a gym buddy. Just try to have some social interactions.