She feels that she will become a man one day

Praise
be to Allah

You have to do a number of
important things: 

Firstly: 

You have to put out of your mind
these negative feelings that are overwhelming you. Allah, may He be
glorified and exalted, created people physically sound and well balanced,
and they do not become unbalanced except when developing problems in
exceptional cases of illness. Most people, however, are moulded in a
precise, proper and perfect way and would not develop any problem except in
cases of real illness that require treatment by specialists. If you are not
really ill, then there is no explanation except that what crosses your mind
is nothing but insinuating thoughts that you should ward off and resist. You
can do that by diverting your attention away from these matters and
reinforcing the opposite, namely true feminine feelings, calling to mind the
blessings that Allah, may He be glorified and exalted, has bestowed upon you
and focusing on obedience to Him. In this world you can never run out of
things to do that are beneficial and there are always genuine opportunities
to attain success, righteousness and piety in this world. So if you really
focus on these beneficial things, and strongly resolve, on the basis of
rational thinking and willpower to reject all thoughts of wanting to be a
man or any idea of changing your gender, then you should know that in that
case you will be following the right course of treatment and you will attain
the psychological stability that you are seeking and aspiring to attain, in
sha Allah. 

Believe us, we are not repeating
something that has no impact, and it is nothing new. Rather it is based on
experience and knowledge, and we are telling you this out of compassion for
you, as the Shaytan is toying with you by means of these feelings. You are
going to be the main cause of your own suffering if you give into these
negative feelings and defend them and justify them on the basis that you are
unable to ward them off. That is nothing but weakness and lack of resolve,
which will lead you to doom unless you take decisive action and resolve to
seek treatment and healing.

Allah, may He be glorified and
exalted, says (interpretation of the meaning):

{But if
they had done what they were instructed, it would have been better for them
and a firmer position [for them in faith].

And then
We would have given them from Us a great reward.

And We
would have guided them to a straight path.

And
whoever obeys Allah and the Messenger – those will be with the ones upon
whom Allah has bestowed favor of the prophets, the steadfast affirmers of
truth, the martyrs and the righteous. And excellent are those as companions.

That is
the bounty from Allah , and sufficient is Allah as Knower.} (an-Nisa
4:66-70). 

Secondly: 

You should consult a specialist
medical doctor or psychiatrist, even if you are certain that you are not a
lesbian. These thoughts that cross your mind may be the result of some kind
of mental illness, and there may be a physical cause having to do with
hormone imbalance and the like, which requires treatment by a specialist. 

It is essential to seek help by
all possible means to resist these negative feelings. So seek the help of a
specialist doctor who deals with fertility treatment and the like (for
hormonal imbalance issues), and of a psychiatrist or psychologist too, for
they may have other means and suggestions that could help you to overcome
this problem, in sha Allah. Do not hesitate to do this, for what matters is
that you should take all measures and be hopeful and optimistic, thinking
positively of Allah, that He may decree healing for you by His grace and
bounty. 

Thirdly: 

Be sure that feelings of wanting
to be a man that cross your mind are sinful feelings, if you respond to them
and behave accordingly by acting like a man, doing masculine things and
being put off marriage because of that. Indeed these matters are the gravest
of sins before Allah, may He be glorified. It was narrated that Ibn ‘Abbas
(may Allah be pleased with him) said: The Messenger of Allah (blessings and
peace of Allah be upon him) cursed men who imitate women and women who
imitate men. Narrated by al-Bukhari (5885). Imitation is a general word
which includes outward appearance, as well as feelings and thoughts if a
person gives free rein to them and does not resist them. Anyone who tries to
resemble the other gender and responds to thoughts of imitating the other
gender, both outwardly or inwardly, is included in this curse and sin before
Allah, may He be glorified. This is quite clear and is something well
known. 

Fourthly: 

Beware of responding to the call
of Shaytan with regard to delaying marriage on the basis of a so-called
sexual problem. In fact marriage is a remedy to this problem, in sha Allah,
but that is on condition that there be a determined resolve on your part to
treat the problem and to reinforce feminine feelings in your heart and to
seek help from a doctor to whom Allah has taught the means of treating this
problem also. In that case marriage will be a means of overcoming the
problem and dealing with it. As for withdrawing and isolating oneself from
natural life, that will only increase you in sickness and will help the
Shaytan against you. 

Fifthly: 

In all these matters seek the
help of Allah, may He be glorified and exalted, by calling upon Him, asking
of Him, turning to Him, humbling yourself before Him and remembering death,
the resurrection and the gathering for judgement, for thinking of death
destroys all temptation and extinguishes all desires. By doing that, you
should be keen to make sure that you read the Book of Allah and ponder its
verses and meanings, and regularly recite the adhkar of morning and evening,
by means of which Allah will protect you from all evil, by His leave. 

For more information, please see
fatwas no.

101169 and

166525 

To sum up: 

The remedy to your problem and
suffering is based on three things: 

1.     Fearing Allah and being
conscious of Him, and being cautious not to overstep His limits.

2.     Strengthening your
willpower and resolving to take necessary measures to treat the problem, and
not giving free rein to your negative feelings.

3.     Taking measures to obtain
physical and psychological treatment to deal with the problem, and
consulting specialists in those fields. 

And Allah knows best.