Croatia is in a pretty safe place, unless Pagans do unusually well or a local Christian power gets a reason to attack you (which is unlikely). If you're playing to survive (as opposed to trying to expand) the biggest threat is going to be your vassals.
Now, if your firstborn living daughter is already married normally (aka not a matrilineal marriage) you're in trouble, as the line will pass to another dynasty and you'll lose the kingdom. If this is the case either switch to ultimogeniture/electorate* succession, or murder the husband and all his heirs. If your daughter is in a matrilineal marriage, the only really problem you'll have is the -26 relationship penalty with vassals you'll get from the female ruler and short reign penalties combined. Also, people will be able to press weak claims against female rulers, which could be very bad or irrelevant depending on who has a claim and if anyone decides to help them. Thus, the most important thing you can do right now is die with a lot of money, so you can hire mercenaries to fight off your own vassals or anyone who tries to press someone's claim on your Kingdom.
*electorate succession is more dangerous because your dukes could vote one of their own into power, but a better choice in general. Ultimogeniture will make your youngest daughter inherit, which is safe unless she's also married, but its a crappy succession law with no long-term benefits.