Four or five girlfriends by age 18 is good going! Don't sweat it, it sounds like you're doing fine. 

Every time you start seeing someone it's like rolling a dice in terms of how compatible you are etc. For every person who finds a really solid relationship after one or two attempts (we all know someone!), there's another who gets 8 bad ones in a row. But the good news is every person you date gives you a little more experience in knowing what you want and how to be with someone you like. You're at an age where everyone is still figuring all that stuff out. You just gotta keep rolling that dice and you'll get there before you know it.

Edit: Try not to make getting a partner define you either! Im a strong supporter of the idea that time outside of relationships is important too. I don't mean seeing people casually, but investing in yourself with hobbies etc. and making yourself a rounded person without "needing" a romantic partner to complete you. The healthiest relationships are the ones that you build ontop of that foundation. Just keep doing what you do and inevitably you'll find someone who compliments that.