There are plenty of major DIY projects that can be done by yourself; I recently re-did a bathroom by myself, my father-in-law building his entire set of kitchen cabinets from scratch by himself, a buddy planed, routed the tongue and grove, installed, and waxed his own floors.
Not having a helper makes things harder or take longer. Truth be told it adds a bit of swearing as well.
Sometime you have to get creative, recently I installed a beam and lolly polls to sure up a springy floor. The beam is 24 feet long and a sandwich 3 2x12's wide with a joint and support poll in the middle. It was installed by running a 2x6 header perpendicular to the floor joist, then attaching a rope sling to the header, then 2x12s where slid into the sling where they were screwed to the header from above (down through the header, essentially hanging from the joists), once all the 2x12s where in place they were timber-locked together, and only then were the polls placed.