How to stay wet?

So my boyfriend and I are highly active in having sex, lately since I started taking birth control I have a kind of hard time staying wet I will be horny and I'll get wet before having sex but then mid way through like half our goes by and I start to dry up, I've had the problem before but I would just get bored. Since birth control I can be in the game and then get dry... It's very frustrating for me any suggestions?