Hi Kate!
It seems like you're in between a rock and a hard place: You've got a sagging floor and, as time goes by, it doesn't seem to be getting any better! So, you've got to act as fast as you can--further traffic, in my opinion, will make it worse and possibly irreparable without immediate corrective action--it's not gotten to that point already.
So, while I understand you are handy, you'll still need professional help. In fact, given the inherent trades involved, I would try to rely on one contractor, and that is a local general contractor or--more often the case to be around everywhere--a local remodeling contractor.
In either case, you need to get plenty of free estimates. I think you know this, but the cost of material and labor greatly varies by location, So, hence, the need for local estimates--that should be given for free.
Out of convenience, here is a handyman site that I come to use, which not only offers informative home improvement articles, but, towards the far left bottom of the following home improvement page, you'll find a bright, kind-of-orange home-guru Bob Villa-endorsed ad that offers up to four (4) Free estimates from local remodeling contractors in your area, whose estimates can be given to you right away, as I understand it:
http://yourhandymanzone.com/Your_Handyman_Zone_How_To_Pages_Home_Improvement_Zone.htm
Once on the above-linked page, scroll down to the bottom to take advantage of the free estimates as a way of easily getting a hold of some local remodelers, and get their free estimates; the rule of them is to get at least three estimates in order to become an informed customer and choose the best contractor in terms of price and quality. Good luck with repairing your sagging floor.
