There is a difference between intending firmly not to do a sin and not doing it.
The condition for accepting the repentance is that when you repent, you intend firmly that you are not going to d it again. If you get weak after a while and fall into this sin, Allah will not erase your previous repentance as it was done correctly and accepted.
Allah will not accept the repentance of people who do sins and claim to have repented though they are intending to do it again after few months. This repentance is not accepted.