I have a similar experience. I have to consciously remind myself that there is stuff happening outside my home that I could visit. I think it helps to realize that usually, no person has to come up with everything themselves. Most other people hear about gettogethers, events, interesting parties or museum stuff through others or being invited as well, so it is a little community effort between you, family, friends, friends of friends, and so on. It just gets difficult when you have almost no one, or none of them.
What helped me is set actual reminders somewhere - can be a post-it on your computer, or something on your phone - that reminds you to go look for things. Check your city's event calendar, check out the local bar's upcoming events, check out what the museums around you have planned for the year, and check the website of the concert halls around you. Maybe there are also organizations in your city offering workshops, or schools/universities offer interesting classes.
Maybe dedicate a few hours to that and put everything you wanna check out in your calendar, even if it is weeks or months in advance.