When prayer is a struggle, do not worry about the prayers that you cannot pray. You yourself are a prayer to God at that moment. All that is within you cries out to Him, and He hears all the pleas that your suffering soul and body are making to Him with groanings which cannot be uttered.
To strive in prayer means to struggle through those hindrances which would restrain or even prevent us entirely from continuing in persevering prayer. It means to be so watchful at all times that we can notice when we become slothful in prayer and that we go to the Spirit of prayer to have this remedied. In this struggle, too, the decisive factor is the Spirit of prayer.