A couple of points that come to mind:
It's always harder at the begining. You rode 2 days the first week, then 4 the second. you may want to back ti down to 2 again this week & if you geel good at the weekend, then take it up to three & wait until you feel good at the end of th week before adding a fourth day, then 5, etc. the important thing is the long-term, not hom many miles you rode this week. It's better to take it slow & do it forever, than to have a great March 2012 & then peter out.

at least around her,e it's been pretty windy out the last few weeks, that will impact your ride a lot. Once the winds die down, it should get easier.

check your nutrition, make sure you're eating well & possibly taking a multi-vitamin, just in case you are low on anything. cut out any crap in the diet, etc. Same thing with sleep, make sure that you are getting enough. You should be fully rested when you wake up. This is extra important at the beginning, while you're still getting used to riding every day.

Most likely you are carrying more weight on your commute than on your fun rides. See if you can tweak your cargo/bike to minimize this weight penalty. I leave my shoes & toiletries at work & just carry a change of clothes. I buy lunch at work, so that also helps. And I'm a fan of carrying as little extra as possible. this is obviously not the case with everyone here & i'ts not 'right' but it does make the ride easier/ more enjoyable for me.

Good luck, hang with it & soom you will be enjoying the rides every day & still feeling great on the weekends.