I second what every one else has said. Take the clipless peddles off. Lower the seat. I have ridden all my life but only recently started riding with clipless peddles. I just didn't see the point. And lets face it, they aren't "necessary." Of course, now I like riding clipped in but it took time. For me, lots of time. It sounds like your goal is to keep up with your DH? If so, eventually you would want the clipless peddles and the seat up - but you are never going to get there if you are too afraid. Have to take it one tiny step at a time - at your own pace. In situations where you feel safe. Start on bike paths at your own speed. Go faster when it feels comfortable.
I applaud you for not giving up. Facing our fears is always difficult but it takes more courage and spunk to keep going and it seems like deep down - you may be more fearless than you think.