So your post was a bit unclear - do you get nervous just riding her or when you start jumping? If jumping - see below to work on the mare sans rider, developing control over the mare from the ground before ever getting on her back to jump.
First work on teaching the mare to listen to voice command from the ground, especially on the lunge line. Teach her that WHOA means halt fairly quickly, and reward her with goodies/lots of pats when she listens. Idea is to get her to listen to the lunger (you) and respond quickly without slamming on the brakes. Once she halts using WHOA from the ground re-inforce it when riding. Word of caution - WHOA means HALT, not slow down. Ideaqlly you want the horse to step underneath itself with its hind legs (to about middle of its belly), then raise the shoulders and sit to halt, with the halt being very smooth.
Once you have a WHOA both lunging and while riding you'll want to lunge the mare over a jump. Start with lunging her over small jumps and be certain to have a ground pole situated to help the horse see where the perfect "take off spot" is located. Suggest you start with small cross rails and have her trot over them (they naturally pick up a canter after they land over the jump). About 3-4 strides after the jump tell her to WHOA.
Repeat until she hesitates about 3-4 strides after jumping because she is expecting the WHOA. Reward her generously when she WHOAs - lots of good girl, pats and a treat. She'll soon figure out that if she WHOAs she gets a goody - providing more inducement for her to halt.
After she calmly halts, accepting the lungers instructions, set up and "In and out" (two jumps with about 2-3 strides inbetween). This is more to get her to sit back between the jumps (thus naturally collecting herself). Don't forget the WHOA after the jump.
She'll probably get too excited to halt quickly when she first starts the "In and out" (just like she will most likely do for the cross rails) so (without jumping on her case) get her to listen to you ASAP. It should get faster after each jump/jumping session.
Once she jumps corss rails quietly, In and out quietly and listens to your WHOA while you're riding her, start riding her over a small cross rail, asking for a WHOA about 3-4 strides after the jump. Carry a goody with you so when she WHOAs you give it to her (from the saddle), pat her and do it again. Soon she'll want to helt on her own to get that goody.
When that happens don't ask her to WHOA until your about 6-8 strides out.
After doing all this, and getting a quieter horse over these SMALL jumps, both you and the horse will be more relaxed. Mare will have time to learn balance over smaller jumps and you'll have time to refine your control of her over the smaller jumps. Both those pieces are missing right now - and her lack of balnce (and natural exhuberance) are why you are encountering problems and getting scared.
If the problem is just riding and not jumps then I suggest you find a very good Dressage instructor and learn how to contro and TRAIN your horse. Once you feel in control you are much less likely to be afraid of the horse and what can happen.
Best of luck. Take a deep breath and relax before you ride next time - and for your sake - don't push it. Flat work will help you and the horse gain muscles, re-learn balance and gain confidence as a team.