Yes, it is sinful, but the case you mention would not be grave matter and not a mortal sin so that due to it you wouldn’t have to refrain from receiving Holy Communion. I recommend that you go to confession and confess venial sins, as that will help you to receive God’s grace and overcome the bad habits such as lying and cheating. That way you can form your conscience and by avoiding minor occasions, you become stronger and will find it easier to overcome bigger temptations later on. It seems to me that you have a delicate conscience and that is good, so long as it doesn’t become scrupulous which involves finding sins where there are none. The best way to avoid sin is to achieve a personal relationship of friendship with Jesus and this by means of primary and penance, offering small sacrifices to the Lord who loves you and whom you love every day.