Should I sell my motorcycle?

You say you 'can sell it for $4500' - are you basing that off of the KBB value, similar listings on Ebay/Craigslist or something - or has someone actually offered you $4500 for it?

Unless you've already got a buyer lined up - chances are you aren't going to get $4500 for it.  Even if someone has expressed interest, there's a difference between talking about it and actually putting up the money for it.

Taking a $2500 hit after only a year isn't a great deal - and if you are still making payments on it, you would have to pay it off before you could just sell it someone else anyway (so whatever the balance of the loan is what you would need to sell it for - unless you've got some extra cash to cover the difference).

I would probably just keep it, get out and ride it more.

It is a hobby bike.  It brings you happiness.  Do not sell it. 

The depreciation is not going to stop, it will slow (eventually). If you don't want it, sell it now rather than let it rot in the garage.