Wants to be Another Man’s Wife’s Husband in Paradise

Firstly: 

It should be noted that it is not permissible for a Muslim to talk to or correspond with a woman who is not his mahram (close relative with whom marriage is prohibited), so it is definitely
not permissible for him to do any more than that, such as meeting her and
being alone with her. … This is the issue of communication between the
sexes, which is the cause of great fitnah (temptation) and evil and
following the footsteps of the Shaytaan until people fall into serious
trouble and major sin, Allah forbid. 

See the answers to questions no.
78375,
34841 and
23349 

Secondly: 

If the marriage contract is done in accordance with the laws
of Allah, may He be exalted, and the husband is the last of the woman’s
husbands, then he will be her husband in Paradise. The correct view with
regard to this matter is that the woman belongs to the last of her husbands,
not the best of them in character, and she does not get to choose between
them. If she only had one husband and there was no one else until she died,
or he died and she did not get married again, then the matter is quite
clear; he will be her husband in Paradise if the two partners are both among
the people of Paradise. 

See the answer to question no.
8086 

Fourthly: 

Our advice to you is: 

1.Keep away from this woman and
stay out of her life, so that you will not be a cause of evil or destroy a
family and cause its break-up.

2.Stop thinking of her, so that
you will not torture yourself with thoughts of something that will not
benefit you, and so that you will not cause yourself illness such as
depression and anxiety, which may lead to physical illness.

3.Carry on with your life and
look for a righteous wife who will look after you and help you with your
religious commitment; make use of your life in doing good and beneficial
things; form a righteous family and give your children a good upbringing.
This is better for both your religious and worldly interests than living in
a world of dreams that will only cause you pain.

For information on the characteristics of the righteous wife,
please see the answer to question no.
71255. 

4.You should understand that Paradise is not a meeting place for lovers! This haram (unlawful) relationship that you had with this woman can never have any place in Allah’s Paradise. The woman has moved on and has entered into a legitimate marriage, so do not act like a social worker searching for a woman whose family forced her into
marriage or who does not like her husband. Do what she did and enter into a
legitimate marriage too. Beware lest you waste your life in something that
will harm you in this world and in the Hereafter. Perhaps Allah will give
you someone better than her instead.

See a similar issue in the answer to question no.
110310. 

And Allah knows best.