Is it normal only interested in one person?

I recently split up with a guy (year long relationship), but we were seeing each other for a few weeks afterwards as it was fizzling out, because we were both not used to breakups and wanted to make it easier on the both of us. What stopped me from seeing him again - and telling him that we should both probably move on and go our separate ways - was getting close and having a little thing with somebody else.

Recently, I really like this new person (I don't think it's rebound feelings, my ex and I never had a very intensely emotional relationship) but this new person is actually very unattainable and a relationship with them would never work out, and we both know it.

I really don't want to be interested in only one person, I want to go out and date and flirt with other people but I seriously have no desire for it, and I would like to also take some time off and adjust to being single, but I can't enjoy it as much because I can't stop thinking about the other guy.

Basically, is it normal to get really into someone I can't have after a break up (he really did help me get over my ex) and how do I make those feelings go away?