can you upload any photos or video?
Seems like both you and your mare are afraid of the obstacle, so in the first place, she tries to avoid it, and then, she is running away from it.
* I don't mean it in a bad way the "afraid". Just that it worries you - maybe.
Since you are now starting to learn to jump, I would suggest to avoid pulling her. It's not sure it's her fault, so do not "punish" her mouth.
Also, if you cannot avoid the speedy-gonzales canter try
1.Try doing 1-2 circles before jumping the obstacle, in trot. Canter should not be an approaching option for any of you two yet.
2. Add 2 trotting poles. Approaching in trot might confuse you or her about when to take off.
1. do not give front your hands. Obviously the obstacle is not that big yet, so don't ease your contact.
1. stay off the saddle, but with as straight back as possible, if you feel that her movement gets you off balance.
2. let her have her 2-3 canter strides and then
3. force her into a circle.
4. keep in the circle as long as it take her to slow down. Keep a medium size of a circle, not too small, or else she ll get off balance, if she is in speed, not too large, or else she won't have reason to slow down.
5. If you want, you can use two short-stride canter poles after the jump, so she ll need to be carefull where to step, instead of just "running away the evil wooden thing" ^_^
PREPARING FOR THE APPROACHING
1. Trot back to your obstacle, but don't let her rush. Jumping itself is not the exercise, it's approaching and leaving it, and she should understand that.
2. Play with your reins. Not too hard, just to keep her focused on something else.
3. Be carefull at the corners, it's the best place for a horse to speed up. If she does, try a circle in slow sitting trot.
As other members have stated many times, jumping needs lot of flat work, so in this way, you combine both.