You can try reclaiming your money from Ryanair... but you may end up spending a LOT of time on the matter. Companies get reputations for a reason. Think carefully how much you consider your time to be worth as an hourly rate and sometimes it's best to treat a modest loss of money as a lesson not to do something (eg paying to reserve a seat with Ryanair) again
Disclaimer - I use Ryanair regularly but normally never pay for any of the extras like seat reservations for a reason