Accordling to the current handbook of instructions (which is as official as it comes!) a divorced person either male or females is not NORMALLY recommended to serve a mission. That means that the door is not totally closed, but it will take a review by the first presidency to make it happen. I don't know the reason why this is the case, but remember that the church has a LONG history of experience in these matters. It stands to reason that many people in your situation have had difficulty fulfilling a mission for one reason or another so the brethren have decided to be cautious in this area.
So when your bishop tells you that it could be "a while" what you most likely need to do is wait "a while". In the meantime, prepare yourself, get a job and earn some money, and be persistent in talking with your bishop about your desires. If you are sincere and if your reasons to go are good enough he might, through the inspiration he is entitled to, recommend you to serve a mission. Then the recommendation will have to follow the regular channels.
There are no guarantees here, just as there are none for anyone submitting mission papers. And, yes, a mission is costly. $400 per month for 18 months, in your case. The church normally requires the missionary or the family to pay that cost. Wards and/or stakes can sometimes help, but that is difficult as it requires others to donate to your mission. If you can earn the money you should. If your family can help, they should. Talk with your bishop about that as well.
I hope this helps. Lord bless you, sister! It seems that your heart is in the right place.
