Hmmm difficult to do that without hurting feelings, but I think you should gently tell her that its not for the best.
i asked my bridesmaids to foot the bill on the dresses! they were more than happy to do that! i bought em a pair of shoes each. smiles all round! lol xx
Tell all concerned that you want them there. Remind them that it's your day and if they can't be civil for it, then don't bother coming. I'm a bit of a blunt Northern woman and that's what I've said in the past. The onus is on them, then.
I'm sure the ladies will have better answers but here is a guy's take on things...<br />
A wedding is a new beginning, a time for looking forward. The wedding party should be people that not only were important to you both but likely will continue to be important to you in the future. You have a casual long distant relationship now with this girl...she should understand how it would be awkward for her to be in the party with her ex as a groomsman (since your husband to be has a much closer relationship with his mate than you do with this girl (I think). <br />
Tell her about this...if she can't understand then that's her problem. A bride shouldn't have to worry about something like this with all the other stresses.<br />
Update: Understand your not being comfortable breaking bad news...I don't even know you...if this girl knows you even one little bit she will understand how hard it was for you to break this news.
Why don't you dig into your pocket, so you don't have to hurt any of them. You can have as many as you want.
2 years away... (OMG!? How do you plan for a wedding two years out?). Put this problem off for at least another 18 months. And tell your Mum "thanks", but it is none of her business is it, really?