Sometimes they begin to behave differently under a new regime. Even hacking out alone may be something that they are not used to - hence the spookiness etc. You could return to your riding school to get some confidence inspiring lessons on another horse . This saves having to travel her in the trailer. If you look in your local tack shop etc., there may well be some adverts for qualified freelance teachers.
Recommendation is always the best advert however so do ask around .
If the mare is a little quieter as she should be as a result of this , then once you have regained your confidence yourself by riding other horses then things should improve , with the help of more lessons at home on her . You may have to face the grim fact that this horse is not the right horse for you under the circumstances, but again you will be able to discuss this after a professional person has made their assessment of the problems too , by riding her themself and seeing you on top too . I hope that this has been helpful to you. Obviously make sure that your mare is not just too full of herself and feed her accordingly. There are many non - heating feeds on the market if she needs extra feed at all now that there is plenty of Spring grass coming through .