Turn up the pressure. When things are down to the wire, both players nerves are running, not just yours. This is the best time to turn it up! I use to get tense and too careful, that's when I made the unforced errors. This is the best time for you to shine, don't be afraid to go for it at the right moment!
I think a lot of people become overly careful. You even see it amoungst the Pro ranks. That actually has surprised me. I tend to do exactly what you are describing. Although, not out of control. Because that can really mess you up too.