That's a good start. As already suggested above, if you take one third of the forsythia stems out now, cutting at the base, yu'll still get flowers next spring and you'll also get new growth which will help balance its shape now iit has more light.
Cut out another third each spring immediately after flowering, making sure you take out the oldest, and your shrub will automatically be renewed every 3 years and thus maintain vigour and improve its shape.
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Cut out another third each spring immediately after flowering, making sure you take out the oldest, and your shrub will automatically be renewed every 3 years and thus maintain vigour and improve its shape.