Issachar, I was born into a semi-Pelagian belief system. Afraid to leave, because of their threats, I stayed years after I knew I needed to leave. I still feel like I am new at being Reformed, so my answers to your questions will be from that perspective.
Regarding sinning "every day in word, thought, and deed", I think I do. The reasons are often not anything specific that I can recall. It's just that I am human: it's that while G-D is perfect in every way, while I am so faulty; therefore, I sin. However, that is not disheartening; my life is in the Messiah, Who stands before G-D, so when G-D sees me, He sees me through His Son's sacrifice, which is enough.
G-D has not "failed miserably" in my life. Not at all. He has done exactly as He intended: He has saved me from my old unbelief and lets me depend on Him fully for any righteousness I can claim. He has shown me that I have no righteousness of my own, that any of my righteousness is as filthy rags, while His is thorough and enough for me.
Of course, I do my best to live as the Bible says. In fact, I am only happy when I do that. It is depressing for me to do otherwise.
Sometimes, He stops me short with His revelation of a sin in my life. This has happened many times. Almost every time, I repent and determine not to do that sin again by His grace and enabling. But I have had times when I balked, a couple times when I went on ignoring Him a couple hours, but He stays on me until I give in to Him. It just does not make sense to do otherwise. Simple as that.
Today, however, I was awakened by the phone at a time when it is illegal for such persons to call. Of course, I knew it was a robocall, but I yelled at the machine and griped, telling it "No, no, no." I didn't look very godly through that minute. I justified my griping by saying, "It's just a stupid machine." So was that how my L-RD would have handled it? No. And anything less than what He asks of me is sin.