Catholics: Am I sinning by not receiving the Blood at mass and only receiving the Body?

Not at all. By receiving the Host you are receiving a complete communion - you do not have to receive both. The glorified Christ is indivisible. To receive him at all is to receive all of him. To receive his body is also to receive his blood, and to receive his blood is also to receive his body. Whether one receives Communion only under the appearance of bread or only under the appearance of wine, one receives both Christ's body and his blood.