I am still struggling to quit sinning. I wanted to know if Allah swt still forgives sins even if we keep committing it multiple times? I am doing my best to stop. I have watched some lectures, I am reading The Greater Sins book and trying my best not to indulge. Sinning has reduced a little, but I am still struggling. Is He still forgiving me?

Also, if this does not work, I was thinking to fast 40 consecutive days as a punishment for not fulfilling my promise to stop by a certain date that I set for myself. What do you think?

If that still doesn't work, do I have to resort to mutah? My parents and I are not in favor of it. Is it even a practical option in my case? Can a virgin male do it if his parents are not allowing it?

Allah is ever Merciful and ever forgiving. If you get the guidance to repent, then know for sure that He is the one to have bestowed you with this tawfeeq and ready to forgive you even if you repeat the sins provided you don't do it intentionally to displease Him and without trying your best to refrain.
Fasting has a great impact on your faith and keeping away from sins so if you can do that, why not. InshAllah you will earn great rewards from fasting and it will keep you away from sinning.
A virgin man can do Mutah with the right woman even without the parents permission if he knows he will fall into haram. Remember to choose the right partner for Mutah which is a marriage after all, be it only a temporary one. Eg a woman who will not harm you in anyway through diseases or one who will keep your dignity even if she’s not a Muslim (Ahl kitab)
Be certain that Allah will surely help you if you hate sinning and He watches those who try all their best and gives them their accord in this world and the hereafter.