Is there any hope for Allah's forgiveness for a servant who watches bad websites, repents, then returns to the sin, and suffers from an inner struggle, despair, and despondency, and contemplates suicide, and feels that he is not fit to be a servant of Allah?

The mercy of Allah is expansive, so do not despair of it. Your despair is greater than your sin. As long as you are a monotheist, beware of abandoning prayer or attempting suicide. Do what you can of good deeds and do not forsake them because of a sin. Repent to Allah, increase in good deeds, and repeat repentance if you return to sin. When you are able to marry, then marry. Otherwise, you must fast, increase in supplication, and keep company with the righteous.