My mum feels left out

I'm not sure you can do anything - you've offered her opportunities to be involved, and she has taken up some of them (e.g. dress fittings) but not others. If her health and work hours are impacting her ability to be involved, it might be worthwhile running any remaining tasks by her, so that she has 'first pick' of what is left to do, but it sounds as if this is more how she feels than her actually being left out.

Sometimes, it can be hard for mothers to accept that their children are moving on. Also, if she is quite traditional in outlook, she may have expected that the wedding planning would be done 100% by your family - it's only fairly recently that the groom's side has become more involved.

All you can do is to keep offering her opportunities to be involved with the planning where appropriate and continue to reassure her of your love for her. Maybe try doing a couple of mother-and-daughter outings between now and the wedding, even if it's just for coffee. Anything to send the message that your mum still has a very big place in your life, even if you are also developing a relationship with your fiancé's family.