You’re not sure about your last confession:
If your last confession wasn’t very good, you might want to be careful about receiving the Holy Eucharist. When i say “wasn’t good”, i mean, if you willfully withheld a mortal sin, you’ve not been restored in grace. You need to see a priest, rectify the problem then you can joyfully receive Jesus. However, if you are scared of venial sins, even unconfessed ones, you needn’t stay away from Jesus. When you pray, God forgives your venial sins. Even though the Church exhorts you to confess them in order that your sense of sin might be stronger causing you to avoid graver offenses with a little more ease. However, venial sins cannot deter one from receiving.
You didn’t observe the fast:
Unless you’re sick or on medication, you must observe the one-hour fast in order to receive.
You willfully came late on a Sunday Mass:
Part of your sunday obligation is attending Mass; and coming to it on time. If you willfully come late to Mass, you do not fulfill your obligation properly. What i normally advise is: If you run late for an early morning mass, wait for the next. However, if you are late for a good reason such as health or some other unavoidable issue, it’s then okay to receive.
It is important to examine oneself to be conscious of Jesus whenever we want to receive. DO NOT LOSE SIGHT OF HIM. Do not approach the altar because you want to be seen, or you want to “appear”. Come to Jesus because your heart calls out to him. Whenever you receive Jesus, let it be out of love and devotion.
“As Catholics, we fully participate in the celebration of the Eucharist when we receive Holy Communion. We are encouraged to receive Communion devoutly and frequently. In order to be properly disposed to receive Communion, participants should not be conscious of grave sin and normally should have fasted for one hour. A person who is conscious of grave sin is not to receive the Body and Blood of the Lord without prior sacramental confession except for a grave reason where there is no opportunity for confession. In this case, the person is to be mindful of the obligation to make an act of perfect contrition, including the intention of confessing as soon as possible (canon 916). A frequent reception of the Sacrament of Penance is encouraged for all.”
– United States Conference of Catholic Bishops