can you send a pic? you say the bottom stitches are long--are they loose? If so the top tension likely needs adjusting. First, unthread and totally rethread the top--making sure you are going through all the tension spots (especially make sure the take up arm is pulling the thread through the check spring of the tensioner). If that is ok, get some scrap fabric and start sewing, adjusting the top tension as you do. That sounds like where you are having issues. It could be your check spring needs replacing--when is last time you had maintenance done?.
If you have changed your bobbin tension, you may want to eae it back to what it was originally so that the tension adjustments on top will work. Problems on the bottom are almost always top tension; shredding thread or weird top stitches usually are bobbin issues. Good luck--nothing more frustrating!