Wale kum salam and thanks for your question.
The commonest cause for blocked nose is deflected nasal septum
,may be congenital or traumatic as in your case.Most probably your nasal septum has got deviated due to accident which you had met and this has led to frequent nasal blockage and nasal resonance in your voice.The drugs your using are nasal decongestants and will give you temporary relief.For permenant relief you need septoplasty
.It is a minor operation which will correct your deviated nasal septum and you will be symptoms free.I would suggest you to consult an ENT surgeon and go for operation.
Hope this answers your question.