I have frequent nightfalls due to which I am unable to perform my fajr prayer during winters because I catch cold very easily and after the nightfall I am very tired. It is no problem during summer as taking bath is no problem at that time. So is there any way I can perform my fajr prayer without going through these difficulties. I have 13 to 14 times per month. I am undergoing treatment for nightfalls for last 3 years but with no success. Please help me

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger. 
It is permissible for a person in a state of sexual defilement to perform Tayammum (dry ablution) under certain conditions:
1) If water is not available;
2) If water is available but due to the cold temperature it cannot be warmed to prevent its harmful effects.

However, if you are able to wash and dry each organ, separately one at a time, then you should do so rather than using Tayammum.

Moreover, you should not delay the prayer from its prescribed fixed time for being unable to use water because the fixed time of the prayer is an utmost condition for the validity of the prayer.

Allah says (what means): {O you who have believed, when you rise to [perform] prayer, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles. And if you are in a state of janabah, then purify yourselves. But if you are ill or on a journey or one of you comes from the place of relieving himself or you have contacted women and do not find water, then seek clean earth and wipe over your faces and hands with it. Allah does not intend to make difficulty for you, but He intends to purify you and complete His favor upon you that you may be grateful.} [Quran 5:6]

The evidence for the permissibility for a person in a state of sexual defilement to perform Tayammum when fearing any harm from using water is the hadeeth narrated by Ahmad and Abu Dawood from ‘Amr ibn Al-A'as, may Allah be pleased with him, that he said when he was sent in Thaat al-Salaasil: "I once had a wet dream in a very cold night and I feared to die if I ever performed Ghusl (ritual bath), so I made Tayammum and prayed as imaam leading the companions who were with me. When we came to the Prophet I mentioned to him what had happened. He then said: "O ‘Amr! Did you pray as imaam leading your companions while Junub (in a state of sexual defilement)?" I said: 'I remembered the verse in which Allah says (what means):{And do not kill yourselves [or one another]. Indeed, Allah is to you ever Merciful.}[Quran 4:29]; so, I made Tayammum. The Prophet laughed and did not make any comments."

This means that the Prophet approved of ‘Amr’s act. This is the opinion of the scholars of the Four Schools of jurisprudence.

As for your complaint that wet dreams bother you, then you should know that this is natural for human beings save the Prophets. There might be many advantages for this and we hope that it will disappear after you get married.

Allah knows best.