This is another way to think about your arms (since I had the same problem). Your arms are connected to your shoulder blades. Think about rolling your shoulder blades up, back and down and dropping your elbows like they have weights on them. This will help you be more relaxed than just bending your elbows.
Think about tucking your seatbones underneath of you- this will help you sit straight and not lean forwards.
Not sure if anyone else would agree, but I think you could benefit from shorter stirrups. You look like you're reaching for the stirrup and your leg slid forward. Do lots of 2 points to get your heel down, and posting without stirrups to get a strong lower leg.