Sitting the trot is tough! I've ridden off-and-on most of my life, almost daily over the past 6 months and can just now sit a big, forward moving trot. I started by learning how it feels (in a baby sense) on a Western jog. Practice sitting an easy trot so that you can relax into it, feel the movement of your hips as you sink into the saddle and the stride rocks you forward, forward, forward. If you really relax into the motion and focus, you can feel each of your hip bones jut forward with the trot (left, right, left, right).
Starting on a working trot will only make you tense up and resist the motion, which is why I say to try this on a jog first.
After you've learned to sit a "baby" trot, you can start to implement that feeling on bigger trots. You won't be able to at first, but as you build up your core and learn to really sit deep, everything will eventually click!