Rear Rim Brakes

Do you have matching wheels as some wheels brake better than others? My summer wheels brake really well but my winter wheels on the same calipers, pads and cables are a bit meh in comparison. 

twist the barrel adjuster anti clockwise to make the brake hit the rim sooner, quick fix if it works 

I loved SwissStop Greens, until I discovered KoolStop Salmons, soft and great in the wet, they disappear quite quickly, but they generally cheaper than than the SwissStops, and better to use up the pads than the rims.

Getting the contact point right on the levers make a lot of difference, especially if you can adjust the levers too.  You want the most force where your hands are closer to the bars (or I do anyway), not so close that the lever is touching the bars or crushing your lower fingers.  A streched hand getting to the bite might seem right, but your hands have less strength there, and can make your hands more tired.

Lots of sound advice here. I'll just repeat those things I've had positive experience of.

I also found switching to SwissStop pads made a significant difference (KoolStop pads also get good reviews at lower cost - although the SS ones do seem to last well, which offsets their price).

Cables need to be clean, lubed and smooth as stated. And as also mentioned, try filing any 'glaze' off the blocks and cleaning any greasy residue off the rim with some white spirit.

Someone also mentioned 5800 (i.e. '105') calipers - I haven't tried those but the preceding generation were excellent compared to the Ambrosio long-reach calipers I'd previously used. But try the cheaper fixes first.

I'd go for changing the inner cable if it's over a year or two old. A dirty damaged inner can degrade brake performance and if it doesn't make things better you can then progress to swapping pads. Also, if you get a new inner cable, you'll be re-adjusting the caliper so you should see if there's any issue with it. Tektro brakes aren't the best, and I can also recommend the TRP ones (they look cool too).

I had similar lack of confidence a couple of years back descending the Gospel Pass, the lack of stopping ability compared to my riding partners was alarming. I had already switched to swissstop pads so decided to upgrade the callipers to Ultegra.

Never looked back.

First, and cheapest, things to check are that

1. the rear brake cable is running smoothly, and then
2. the pads are close enough to the rim that the brake hits the rim within the first half of the brake lever's movement range

If those don't help, then get new pads. If this doesn't help then buy new brake calipers. My bike came with cheap Tektro brakes which were terrible. I needed 'long-drop' brakes and so bought a set of TRP RG957s (https://trpcycling.com/product/rg957/) which were expensive but which are really excellent.

As above, cable drag (inner and otter cables) can really degrade a rear brake, also rear brakes have a harder time with road spray and muck getting into the pivots ....if your feeling up to it take them off (along with the pads) and give it a good clean and Re-lube getting into the pivots 

Assuming...right or wrong this isn't a new bike.....if so chances are it has Galvanized cables....often....well 99% of the time that is a problem...with galvanized when water ingress of the outer cable lays inside, the cable corrodes and "swells" so then drags as it slides through the outer....swap out for a stainless steel cable... you'll get a cleaner pull....for more bite after.... Better pads than stock....

Some Tektro brakes are ok, some are very poor. Calipers can make a surprising difference (my 5800s @ £30 gave a massive boost over an older Ultegra set, but they aren't direct mount). However, the first thing to try is always the pads, my favs are Swisstop Pro BXP. They are not cheap (£25) but that is for a set of 4. They provide smooth excellent braking in all conditions and last reasonable well. Other alternatives are out there but swapping from Tektro pads would definitely be my first step since there is no reason to put up with a poor rear brake (I can lock up the rear wheel on all my bikes, running a variety of wheels and brakes).
If you want to do some quick maintenance though, just pop the wheel out, pick out any embedded debris in the pads, resurface using a file (not sandpaper) and see if that helps.