Most people plan a cycling commute the way a satnav would: shortest distance, fewest turns, straight down the main road. Then they do it twice, find it unpleasant, and put the bicycle back in the hallway.
The parallel street
Almost every arterial road in a city has a quieter street running within a hundred metres of it, going to roughly the same place. It is usually thirty seconds slower. It is also, generally, the difference between arriving relaxed and arriving braced.
Spend one weekend morning riding your commute deliberately badly — take every wrong turn that looks appealing. You will find two or three of these streets, and they will change how the journey feels for years.
Ride the timing, not just the route
Leaving ten minutes earlier often does more for a commute than any change of route. Traffic in most cities thickens very suddenly rather than gradually, and being on the far side of that threshold is worth more than a lighter bicycle.
Stop treating it as training
A commute made at eighty per cent effort is a commute you will need a shower after, and one you will start avoiding. Ridden gently, in ordinary clothes, on an upright bicycle, it stops being exercise and becomes transport — which is the only way it survives contact with a busy week.
Learn one junction properly
Most routes have a single genuinely awkward junction, and most riders approach it slightly differently each time. Work out the version that feels safest — which lane, which lights, whether to take the pavement crossing instead — and then always do that. Removing the decision removes most of the stress.
The point
A commute you enjoy is one you keep making. Everything else — the bicycle, the bags, the lights — exists to support that, and none of it helps if the route itself is miserable.