I felt led to answer your post today not because I'm an expert on the subject, but because I was like you several years ago when I was trying to learn and practice speaking with other tongues. When I struggled with this issue a while back, I began to be caught up in the actual procedure of doing it as opposed to how or why the spirit led me to do so. During this time the Lord pushed my understanding towards speaking in tongues by FAITH!  A very well known evangelist provided this simple truth by way of radio while driving home from work one day. This was monumental in my continuance to speak with other tongues in my prayer life even when I wasn't "feeling it" so-to-speak.  Our faith in Yeshua as messiah is based off the very same faith.  Just because we're not in tune with His path, or not having a good day, or don't understanding something doesn't change the fact that He is still LORD, God, our Father, and the Creator!  Being led or drawn by the Spirit of God is how we should be, though this became easier for me after much growth.
 
After faith in Yeshua (or for some right at their conversion) men spoke in other tongues according to when the spirit of gives utterance. Examples include: On Shavuot (pentecost) it occurred as the Holy Spirit descended into the upper room birthing the church age.  Others in the book of Acts did so after their immersion from the Holy Spirit during prayer with the apostle Paul and other followers of Yeshua.
 
Always know that tongues give us a perfect way of communicating with God that bypasses the barrier of the flesh remembering that the holy spirit instructs and gives us this boldness.  Right prior to the stoning of Stephen, we all know he spoke to the Sanhedrin regarding Yeshua's life, ministry, and purpose.  Somewhere in the point of the story vs. 55 of ch. 7 says that "Stephen, being full of the Holy Spirit..." and again with Paul in Acts 13:9, " Then Saul, also known as Paul, filled with the Holy Spirit stared straight at him and said...".  So in these instances, these men were moved by the spirit of God to say exactly what was needed at the time.  The same is true for tongues.  You already seem to have a strong trust in speaking it, but are concerned with the details.  Don't be so much.  As long as your intention is to give Glory to Jesus and His mission, I promise you you're not sinning in doing so.
 
I remember one time I felt led to speak in tongues back in the beginning of my understanding of it in my bedroom after studying and praying.  When i first started, the first few syllables felt like gibberish.  After about 30 seconds of me continuing to speak in the spirit not yielding to doubt, a language flowed out of me that sounded somewhat like Spanish. However, this is not a necessary component that "validates" your experience. Your validation comes by way of the Word of God.  So bottom line...keep it up.  The bible already clearly states that these signs shall follow them that believe (Mark 16:17).  Trust the Lord that he will anoint you to speak to Him in your heavenly language when you need to.  Have faith!
 
In Yeshua,
 
Josh List