Is a female student obligated to abandon a health specialization in which she feels pressure and regret, after having previously sought Allah's guidance (istikharah)? And how can she find psychological comfort and supplicate to Allah concerning this matter?

Your feeling of regret does not mean that your Istikhara was incorrect. Goodness may lie in continuing with your major or changing it. Seek Allah's help and ask Him to inspire you with wisdom and to facilitate what is good for you in your studies and all your affairs. Consult those with experience. Be easy on yourself and do not exaggerate, for changing or continuing is not a cause for worry. You must be patient and content with Allah's decree and destiny.