I've experienced that too at my place - a cold apartment in winter really is unpleasant, especially when you're freezing at night. Legally, your landlord has pretty tight limits here. He has to guarantee you a minimum temperature, and 19 degrees is way too low. The courts see it this way: during the winter months, the heating has to work in such a way that you can reach at least 20-21 degrees during the day - at night it can be somewhat lower, but not extremely. If it gets significantly colder at night in your place, your landlord is violating his maintenance obligation.
What you can actually do: document the actual temperatures in your apartment, preferably daily and in different spots. Then write your landlord a written complaint (letter or email) that the heating isn't functioning adequately and isn't maintaining the temperature. Give him a deadline to fix the defects - usually 1-2 weeks. If nothing changes, you can reduce your rent or, if necessary, withhold the security deposit until the problem is solved. In more serious cases, a tenants' association can also advise you - most cities have one and offer free assistance.