The previous poster is right. These matters should be discussed in the confessional priest. If possible it may help to find a priest who has a good handle on moral theology and can advise appropriately.
It helps to properly understand what a sin is and why something is sinfull.
It helps to understand the difference between:
Mortal sin Vs Grave Matter Vs Venial Sin.
This is my understanding:
A sin is something which will damage your relationship or ability to have a good relationship with God. it harms or is likely to harm the gifts God has given to you and those around you.
1) It directly harms or is likely to harm the people around you. (murder, drunk driving, gambling with the families grocery money)
2) It harms or is likely to harm the gift of your own person (body mind or soul) (self harm, actions likely to cause addiction, very risky actions),
3) it prevents you from serving god and your fellow man: (actions which lead you into some form of slavery: entering into large debts, letting yourself become addicted...)
4) It directly offends God and your ability to love Him and accept his Love (blasphemy).
Very few sins other than blasphemy and sacrilege fall into category 4.
Almost all sins are things which harm the sinner, people around them or the society in which they live (thereby harming many people)
When you receive the Eucharist, you receive the Body and Blood of the Crucified Christ. This is the flash and blood of God's own Paschal Lamb, sacrificed to pay the ransom for our sins. We MUST eat of His Flesh and Drink his Blood to be Redeemed. This is part of the necessary mechanism God has given us for our Salvation.
The Eucharist therefore is a mechanism for the Forgiveness of Sins:- but not Unrepented Sins -
To receive the Eucharist, knowing that you have committed acts that massive harm your ability to Love God is a bit a man punching his wife in the face and then still expecting her to fulfil her "marital obligations" without first repairing the relationship. - The second act - instead of fulfilling its unintuitive function is a further sin - it may very well be rape.
However coming back to the definitions of Grave Matter, Mortal Sins, and Venial Sins:
Many things are Grave Matter. (serious sins which have the potential, as a single act, to shatter your ability to accept the love of God, and therefore condemn you to Hell).
A Mortal Sin, is when you commit a sin which is Grave Matter, knowing it is a serious sin, Doing it with full freedom of the will. When this happens you have deliberately and voluntarily committed a sin which you cant say "Sorry I didn't mean it". you did. you did it on purpose, knowing what it would do to your relationship with the Lord. (and the person or people you probably hurt by the act).
This therefore requires a full and proper act of contrition. you must genuinely be sorry. you must say sorry. To God, and to the people affected. You must make amends where that is possible.
Lets say you broke into someone house and stole their brand new 50" TV that they had just bought for the family for Christmas. You have hung it on your own wall. Can you go to Holy Communion and expect that by taking the Eucharist you are being forgiven for the Theft and intrusion into the other persons home, while the spoils of that theft are hanging on your own wall? - that is madness. you are not sorry - you cannot accept Gods forgiveness - you would be lying. receiving the Eucharist in that situation is the spiritual equivalent of Rape as described above. (not exactly but that's the limit of analogies and metaphors).
To be forgiven for the theft by God, the thief must 1st give back the stolen property. 2nd repair the damage to the house they broke into. 3rd apologise to the victims, and attempt to make restitution for the emotional harm caused. this may include facing the legal consequences of the theft (prosecution, conviction, civil punishment, possibly jail) 4th apologise to God, in confession and receive absolution.
5th and last, return to the Altar and receive the Body and Blood of Christ in the Eucharist.
you cannot short circuit this and jump to stage 5.
On the other hand: If we look at the Anabelles's example: using Artificial Birth Control. and through no fault of your own, not knowing that is is wrong or why it is wrong:
in this situation engaging in the marital act while taking the pill for birth control reasons is sinful. but not knowing this if the person were to come face to face with God and asked to account for their sins they would probably be shocked at the harm that sin had caused, and be truly repentant and contrite. It would almost certainly not make it impossible to accept Gods love and forgiveness it would not send them to hell - (but would probably be a painful realisation and need to be purged (in purgatory).
For Anabelle to receive the Eucharist in this situation would probably sadden The Lord, as she is unaware of the need for repentance, but is not in itself a deliberate, sinful action. in this situation the Lords forgiveness does flow through the Cross. Through the body and blood of Christ. and will wash her clean.
Read the information in the catechism. there's an on-line version on the USCCB website which is searchable. there's also another on-line version on the Vatican website.
Make an act of contrition, and resolve to go to confession as soon as possible. Think like this: if you got up from your computer, and were in an accident on the way to work or the shops do you think you would be able to accept Gods forgiveness and go to Heaven? If not then you should be on the phone finding a priest who will see you ASAP. Otherwise you should try to get to the next available confession at a scheduled time.
Most priests will hear confession after the Daily Mass.