Particular (adj.):

1. exceptionally selective, attentive, or exacting; picky.

Maybe it shouldn't be that we don't let ourselves trust anyone. Maybe we should just be more selective when it comes to who we let in. Maybe we should take a look around us, at who has really been there, and realize that no, it's not that we have trust issues, it's just that we know who we can trust... and that those select few are more than enough. :)