If you had solicitors acting for you at the time then you should consult a new solicitor to examine whether you have a potential action for negligence against your former solicitor. If what actually happened is that you did not use your own solicitor but instead signed paperwork that was prepared by your ex husband's solicitor then you are in a much more difficult position.
Also, it not quite so simple as not claiming the pension share because the pension share would be wiped out by the costs for which you are liable. The two are separate and can only be linked if there is a formal agreement to that effect. If there is no such agreement then each claim is free standing. It is also perhaps worth pointing out that if you have not paid the costs then they will be entitled to interest (which is likely to be at the rate of 8% per annum since 2019).
You need to take proper legal advice on your situation in order to decide what, if anything, you should do from here.