It seems like you’re overthinking it, trying to force your arousal. Let it happen more naturally, slowly. If you’re feeling horny, let your mind wander a bit – think about things that turn you on. Daydream a bit. Maybe scroll through a sexy tumblr ;). Read some smut/erotica. Watch some porn if that appeals to you. Let your body settle into a state of arousal before you set out to go from 0-100 and masturbate instantly!
Put yourself in an atmosphere that’s quiet and relaxing. Turn on some music, low, if that’s something you like. You want to feel safe and comfortable, without fear of interruption (if you can). Try taking a soothing bath to relax you. Think of it as a slow ease into arousal.
It’s all about your frame of mind. If you’re feeling relaxed, comfortable, and you have time to just close your eyes and daydream about whatever it is that turns you on, that will do the trick. If your head is kind of cluttered that day and you need that extra boost, porn – whether read or watched – can help.
Don’t force it. Don’t overthink. Let your body take its time and react naturally to whatever stimulation is provided. Also, nothing wrong with humping a pillow, but you can check my sex advice tags for more tips on masturbating if you want.