He divorced his wife because she was mentally ill, but he is still emotionally attached to her and wants advice

It is part of Allah’s decree for
His universe that He has made this world the realm of troubles and problems,
sickness and separation, exhaustion and tiredness, as trials for His slaves,
to see whether they will be patient or impatient, and so that they will long
for the Hereafter, Paradise and that which is with Allah, where there will
be no toil, distress, exhaustion, hardship, grief, death, sickness or
separation. This is Allah’s decree for His universe, and the way of Allah
cannot be changed. 

In your words, is the answer and
remedy for what you are faced with. Your relationship has come to an end
with separation and divorce, and you do not intend to take her back. She is
sick, suffering from a mental illness with which it is impossible to
establish a normal marital relationship. What appears to be the case is that
you tried but you did not succeed, so what comes next? Why hold onto a rope
that has already been cut off? Why live in the shadow of wishful thinking
and impossibilities? 

You gave a clear answer to these
questions, and we appreciate your openness: “my heart wants to take her back
but my head says no.” This is the entire story and the entire solution. The
heart is inclined, but reason corrects you and prevents you from stepping
onto that slippery slope. 

But you have to give precedence
to strengthening the resolve of your mind, so that you can forget about her
completely. That may take some time, but in the end it will come, according
to your strength, resolve and certain faith that Allah will compensate you
with something better than her and will compensate you for the calamity with
something better and will reward you for what you have suffered. 

Ibn al-Qayyim (may Allah have
mercy on him) said: 

If there is no way for the lover
to reach his beloved, either because it is not divinely decreed for him, or
because of some shar‘i impediment, or both, then this is an insurmountable
problem. Part of the remedy is to make himself lose hope and despair (of
ever reaching his beloved), because when one loses hope in something, he
will be relieved of worrying about it and will stop paying attention to it. 

If the sickness of love
continues even though he has given up hope, then this is a sign that he has
developed some serious psychological problems. In that case he should move
on to another remedy, which is to address his mind and think rationally,
because it is known that being emotionally attached to that which one has no
hope of attaining is a kind of insanity; the one who is afflicted with that
is like one who falls in love with the sun and thinks constantly of climbing
up to it and running in its orbit with it. Such a person is regarded as
insane by all rational people.End quote from
Zad al-Ma‘ad, 4/251 

You are still young, as you
stated in the details you gave (that you are thirty-four years old). It is
not fair, it makes no sense and it is not wise to spend your life regretting
what you missed or something that cannot possibly come back under any
circumstances. Look for another wife who is suitable for you, and start a
new page in your life. Life does not stop because of separation or failure,
and nothing could make you forget a woman except another woman who is more
suitable for you. 

Ask Allah for guidance (istikharah)
and look for a wife who will bring you happiness, and cut off all ties with
your ex-wife, once and for all, because she is no longer permissible for you
now, as you are divorced, and there is no point in keeping in touch with one
another on the phone. If you can change your phone number, then do so, so as
to stop her calling you and causing you distress. 

It should be noted that
forgetting may take some time; as we said, that depends on your strength and
determination. Ask the Controller of the hearts to remove from your heart
whatever is left of this relationship and ask Him, may He be glorified, to
reward you for your calamity and to compensate you with someone better than
her.

We ask Allah to grant you peace
of mind, a bright future, and a righteous wife who will compensate you for
what you have missed, and that Allah will grant you the reward of those who
are patient.

And Allah knows best.