Write what you know! It's perfectly normal, just be careful and pay attention to see if you're not creating too many Yous, or if your stories don't end up having the same themes explored over and over.
As for Splitting Image - boy, pretty much every major character is either a piece of me or someone I know. My therapist actually asked for some brief descriptions of the characters and went on to list how much it's from my experiences, so I'm at the same time proud but disappointed at how easy it is to find
Mortimer has self loathing issues(if he's not helping someone else or working towards such, it's horrible and he isn't doing it right fast enough), he constantly thinks back to old mistakes like they're still the current status, he has intrusive thoughts, he's got it in his head he has one shot at anything and shuts down if someone makes him stressed over it. So, uh, yeah.
Veriesin gets angry, gets frustrated at being angry, gets angrier, says something rash, regrets it later. Spinelthorn is overprotective and also distant because he's afraid of hurting the person. Berylclaw had a 'polite girl' image and hesitates in standing up for herself and looking rude. Elise is quiet at first, talks a lot later, regrets being so open so quickly.
I could probably go on with every character if I could remember all of them right away.