I m a married women.I had sex with my boyfriend who is already married. Now we both are feeling shameful and discontinue
Will Allah forgive me for our sins? Do I have to tell my husband about this?
First of all the sin you've both committed is among the worst you may do and the fact that you feel ashamed is a good sign as it means both of you still have faith and know that Allah sees us while we can't see Him.
Also note that the sin you've committed has two parts, one that you need to repent from from Allah and this needs sincere repentance: sincerely regretting it, stopping it at once and stop any contact to your co-sinner and ask Allah for forgiveness.
Note that in a hadith qudsi Allah says:
Three (are the persons) with whom Allah would neither speak, nor would He absolve them on the Day of Resurrection.
Abu Mu'awiya added: He would not look at them and there is grievous torment for them: the aged adulterer, the liar king and the proud destitute. ([Sahih Musliam]1)
Note that "aged" in this context also refers to "married" as a young adulterer usually would be unmarried.
As for the part you may ask forgiveness for by Allah, Allah may or may not accept. But we know that:
Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah . Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful." (39:53
So the more important question now is: Should you tell your husband, as it actually is the second part of your sin: You first disobeyed Allah and committed an awful sin, and as a married person you abused your husband's trust and had an illegal relationship with somebody else.
As in the shari'a the sound condition of the spousal home is important scholars say that legally a married woman who committed zina has two possibilities, the first is to get her legal punishment by either admitting her crime in this case her and her husbands home will be demolished and the trust is gone and maybe children would be left alone without a caring mother or she keep it hidden hoping that her good deeds on the they of judgment may clear her register in front of her husband whom she cheated in this life so that he might forgive her in the hereafter.
So concealing your sin is the better option in case there's no reason to report to your husband (pregnancy due to zina). Based on ahadith such as:
The servant (whose fault) Allah conceals in this world, Allah would also conceal (his faults) on the Day of Resurrection. (Sahih Muslim)
"Avoid these filthy practices which Allah, the Almighty has prohibited. He who commits any of these, should conceal with Allah's Most High Veil (i.e. should not speak about it), and should turn to Allah, the Most High in repentance, for if anyone uncovers his hidden sins (to us), we shall inflict on him the punishment prescribed by Allah, the Al-Mighty." (Bulugh al-Maraam)
Some scholars consider it as necessary that a wife who cheated her husband pauses her physical relationship with her husband for at least one menses to get clarity whether she is pregnant or not. This is also possible by visiting a gynecologist these days.
You shouldn't force yourself to tell anyone if you don't think that that's a good idea. Having sex is your decision to make and you shouldn't feel ashamed for doing what is essentially a very natural thing that billions of peoples do everyday. Your world is not destroyed simply because you had sex and Allah will not forever judge you to be evil. I believe that Allah looks at the more important things happening, such as why you had sex and judges you on your intentions as opposed to the act itself.
For example if your relationship with your husband has broken down and/or he is some way or the other abusive and you found comfort in the embrace of another man than Allah will most surely understand and judge you based on your circumstances. We all look for people who care for us and are willing to love us. Seeking and wanting that is not an evil in the eyes of Allah.