Finishing a spell on the next turn due to slow

[holywhippet]

When under the effect of the slow spell, if you try to cast a spell with a casting time of one action there is a chance the spell won't be finished until your next turn. If this is the case, does this follow the rules on casting a spell with a casting time of longer than 1 action? ie. Are you now forced to maintain concentration on the spell until it is finished and does that mean you automatically lose any spell you might already be concentrating on?