I'm trying to design a frame for a Baja vehicle (go kart, essentially) in a 3D sketch. It will be largely symmetrical from left to right, so my plan was to design one side, then mirror it to the other side. Getting the first half sketched was tricky, but doable, but when I went to mirror it I found that function wasn't available for 3D sketches, so I did it manually.

Although I was able to exactly define one side of the frame, mirroring it to the other side, connected by lateral crossmembers, is proving impossible. No matter what I do, when I put in the last few relations the sketch becomes overdefined. I've tried setting each line on the new side equal to the corresponding line on the old side, and I've tried defining the new side exactly the same way I defined the old, but nothing works. Frequently, the sketch will even go from underdefined to overdefined when I delete a relation/dimension/line. It makes absolutely no sense.

I've spent hours trying to work this out and am at my wits' end. Could someone help me find my problem? (I'm hoping it isn't just an issue with SolidWorks.)

I'm using SolidWorks x64 2014 student edition.