If you're often plagued by indecision, make a pact with yourself t0 default to "yes". (Or you could just ask Shonda Rhimes.) In other words, you should accept invitations if you're at all undecided about whether or not you want to go. Indecision is frustrating for everyone involved. As the undecided person, you feel anxious and pressured to commit. Meanwhile, the host is annoyed by the lack of a concrete plan.
A defined system for responding to invitations will help you to better manage expectations, prevent you from wasting time agonizing over whether to go, and push you to become more open to new experiences. After all if you've never done it before, you may end up actually enjoying it.
Defaulting to "yes" will push you towards a more optimistic outlook and make each "yes" you say feel less consequential.