Allergies can become very irritating and difficult to manage. There are many ways to decrease the effects of allergies, especially symptoms related to sinus congestion. As you mentioned, there are many over the counter decongestant medications that are very good at relieving the sinus pressure and congestion associated with your allergies. It is important to note that some of these medications can make you drowsy and so they should not be taken when operating motor vehicles or heavy machinery. There are medications that are made to be taken during the daytime, and these usually contain caffeine, and so they are not recommended to be taken just prior to going to sleep.
There are also nasal sprays, menthol based ointments that you rub on your chest, humidifiers and prescriptions medications that are also very helpful in opening up your sinuses and relieving the congestion and pressure. Of course, it is recommended that you be evaluated by your primary care doctor
who will be best able to evaluate your medical history, current presenting symptoms and decide upon the appropriate work up and possible treatment options if deemed medically appropriate. With the appropriate treatment regimen, there is a good chance that your symptoms may resolve.