I cannot fast due to hard work

Hamidan wa musalliyan, First of all, you need to evaluate your dependency on your work and your financial difficulty. If you can find another equally decent job and then may observe fasts, then you should change job. But if there are no stable work available and nothing sustainable, then shariah has prescribed Qadah for such persons.

The shariah has prescribed qadah(replace later) for someone that find himself in critical physical weakness that will prevent him to do his job. Severe hunger, severe thirst that may cause physical disability or cause inability to work are valid reasons to postpone fasts.(Bahr Raiq).

(Note that Fuqahas has opened this permission because it is haraam for one to ask or beg when you are healthy to work. So much so that the person is allowed to postpone fasts if fasts prevent him from halaal earning! (Shami))

So if these physical critical conditions are found, then you may fast in cold seasons whereby your fast will be shorter and bearable. But if you have still same difficulties throughout the whole year and your job career is set same, then you should give fidyah. Give the value of a sadaqatul fitr for each day of Ramadhaan.(Dourre Mukhtar)

And if someone is financially unable to pay fidyah because of financial contraints, then he should do istighfaar.(Raddul Mukhtaar)
Bear in mind that this permission of postponing fast and not fasting is only applicable to those critical conditions mentioned above. One should know that it is farz and obligatory to observe the fast of Ramadhaan in normal conditions.
The crux of this masla is derived from this ayah:

أَيَّامًا مَعْدُودَاتٍ فَمَنْ كَانَ مِنْكُمْ مَرِيضًا أَوْ عَلَى سَفَرٍ فَعِدَّةٌ مِنْ أَيَّامٍ أُخَرَ وَعَلَى الَّذِينَ يُطِيقُونَهُ فِدْيَةٌ طَعَامُ مِسْكِينٍ فَمَنْ تَطَوَّعَ خَيْرًا فَهُوَ خَيْرٌ لَهُ وَأَنْ تَصُومُوا خَيْرٌ لَكُمْ إِنْ كُنْتُمْ تَعْلَمُونَ

Faqat, Wallahou A`lam.