Feels like I can't initiate sex

I've been with my girlfriend for 8 months now. First relationship for me. For some reason it always feel like when we have sex, it's because she initiated it. When we go to bed and I'm in the mood, I kiss and caress her and just try to initiate. If I feel like she's not responsive I give up. The thing is that she is rarely responsive when I'm the one initiating. I'm stuck in the mindset that if she want to have sex, I will easily know. So when we don't have sex I'm not sure if it's because I missed her "signal" or I didn't try hard enough or that she's just not in the mood. And then I get frustrated with myself. Even when we flirt all day long, when the night come there is a big chance she won't make a move or I give up trying getting things started.

Maybe I give up to quickly? Maybe I could just ask her if she's in the mood? Any tips from more experienced people would be great. I'm kinda lost.