Work without stirrups a lot, let your lower leg sink down. Improve your balance and also muscles - ride in posting trot WITHOUT stirrups. Once you'll put the stirrups back on, they will feel short even without shortening them before.
As somebody suggest shortening the stirrups first - this is just a quick fix and probably will cause you other problems later, such as going forward with your shoulders.
What I did was ride with short stirrups to get my heels forced down , then I gradually lengthened my stirrups and now my heels are always down and are stable on the horses sid
Working with no- stirrups for 10 minutes or so can help. Once you take your stirrups back your legs feel longer and sink into the stirrups more :)
riding standing up does help a lot. because i used to have this problem myself.
Ride standing up in all the paces to get your heels down and riding without stirrups to open your hips and take knees away from the saddle to get your leg on. It will make your leg more secure and effective
Maybe I got this wrong...but would it help to get your stirrups a bit shorter? I always feel safer with shorter stirrups :3
Work in two point a bit and you can try thinking that your heels a digging in the sand. Also, you can stand on the stairs with your heels down and that might help