Don't give up! That is the most important thing first. We all go through frustrations finding motivated sellers as I too fell victim to this discouragement. Many others initially make it seem to newbies that it is easier to find motivated sellers then we actually come to find out during our marketing efforts. In any event it takes much work and trial and error in life to find the right recipe that works best for you. I recommend taking some time to mentally decompress so you don't quickly get burnt out. Then maybe try expanding your marketing efforts to social media via Youtube videos, FB, Twitter, etc. Also if you write the same message on every letter then maybe try altering what you put in the letters to see if that works better for you. You can also try driving for dollars which is basically driving in areas and picking out properties that look distressed. There are many ways out there to market and you can always create your own way. What works for one person will not always work for others depending on the context and other variables of the situation.
The old saying goes "there is more than one way to skin a cat". So don't allow yourself to get angry and frustrated, just use that as ammo to keep trying until you reach your goals. Every failure we endure brings us one step closer to a success.