Don't be ashamed to make mistakes in French, you are not alone. According to a survey realized by le « projet Voltaire », and published in the French daily, « Le Parisien », on 15 June, my dear compatriots could master only 45% of spelling rules in 2015 against 51% in 2010.
A terrible decline states the weekly, « le Nouvel Observateur » and which can be attributed to the national educational policy which substantially suppressed the number of hours dedicated to French tuition favouring other more modern subjects. Another reason may be « sociological changes » induced by the use of the keyboard of smartphone and computers.
But some differences within regions and genders are quite interesting. People in the South, Languedoc Roussillon and Pyrénées-Orientales especially, have better spelling and grammar. Though it seems that the candidates were not from equal educational background, the better results in the South may also be explained by the fact that there are simply better teachers in the South : if you want to get a job in the South, where everybody wants to work, you just need the best marks.
Another discrepancy is that 48% of women seem to master the grammatical rules against only 43% of men. According to the initiator of the survey, Pascal Hostachy, women would read more chick lit (sic) and would more willingly admit this weakness (which I believe) and thus improve their poor spelling more easily, which is sensible.
At least now I know why I receive less love letters than in the past (- :
Here is the website where you can test your spelling and grammar.