The best way to increase your speed over training rides is to ride with someone who can ride a little bit faster than you and then just sit in behend them.
Don't do any work on the front, let them keep you at a steady pace about 4-5km above what you would ride on your own.
I do this with my partner and sons - for them its a comfortable recovery ride, for me its a solid, exhausting workout.
Do this kind of ride 1-2 times a week.

Do a slow ride 1-2 times a week - this is called active recovery and it is vital that you do not make every ride a 'race' and try to do them hard and fast.
So what is a slow ride?
Well, I road-race at about 32-35kph (as long as i can stay with the group)
I time trial at just under 32kph
I ride at a recovery pace at 24-26kph.

Once a week do a long ride... 20m, then a couple of weeks later 25m, then 30m and so on. The "rule" for increasing distance is no more than 10% a week.

Remember you are asking your body to make a metabolic change. You want it to respond differently to stress at a cellular level, and this takes time. Muscle grows in response, your heart learns to react differently, weight is shifted around your body or sometimes shed completely.

Be patient with yourself, and if you want to measure yourself and see your improvements, then choose a course you train on and every few weeks (when conditions are similar -weather/wind/light/temp etc) try and ride it at a good pace and compare that time. Try to avoid comparing every ride you do with the previous. This is an unhelpful habit in cycling as the conditions for each ride are different and it means that you often forget to do the all-important slower riding.