Hi there, I just want to let you know that you are not alone on this. I am struggling sexual orientation also.
There are many reasons why one believe that he/she is a gay. Each person has their own reason. Some have lust for a nice body, some lust for sex organ, some fall emotionally for a guy (eg. close friends), some come from broken family (eg. parents divorce, single parent), some have same sex confusion (eg. molested by a male guy), some have early stage of dreams of same sex, addicted to pornography, masturbate too much and etc.
I believe that there could be environmental and biological reason that affect one's sexual orientation. Perhaps it could be due to genetics (some one in your family trait pass it down long time ago), brain structure and hormone imbalance before birth/when grow up, or exposure to same sex environment, internet pornography exposure and etc.
Whatever the reason is, I do believe no one PURPOSELY choose to have this feeling. Most are developed unconsciously and unintentionally, over a period of time.
From the christianity point of view, human are born with sin. By following god's guideline, you may be free again and reborn as a new person. You can approach "Choices" (Church of our Saviour, Queestown, http://www.coos.org.sg/choices/index.htm) to seek some help. There are some gay people who seek help there to changed their life around. Of course, there are also some who left and went to "safeheaven" because they failed to change. Another useful website would be http://www.peoplecanchange.com/.
From the buddhism point of view, it may be due to bad karma (cause and effect). Perhaps we have done something bad in the past (eg. sexual misconduct in the previous life) and hence we have to learn a lesson in this life. Bad karma can be overcome if we are aware and englightened, and not in the state of ignorance.
I can not comment much whether one can change successfully or not. But sometimes you really have to sort things out what is the reason you are changing. What you want in life? Btw, what is the meaning of life (to follow the same pattern like the other people do?)?
Is it to conform to society's pressure? Is it to live to the expectation of the others and sacrifice your ownself? Is it to please your parents? Is it just to make sure you can pass NS and then go back to the old path after you NS? Is it you really think the lifestyle is not suitable for you?
Btw, I have heard some gays don't declare 302 in NS and they did so well that they became officer, specialist and regular.
Also, do you really love a gal before you get marry? Even marry can divorced. Even marry not necessary can have kids (some don't want, some unable). Even have kids, does not mean happiness. I hope that you are aware that some married guys even seek sex from guy, without their wive knowing. Some people remain single/celebate and live in the fullest of life, surrounded by friends,
though not partner.
Perhaps if you really want to change, you have to surrender and let go your desire and lust. Throw away the same sex thoughts when you ever see a guy. Don't be like a stubborn ghost who always want to seek revenge, who don't want to go reborn and stay at one place forever (just an analogy). Let go, let go and let go.
You can also start tracing why you think you are gay. Break the reason into pieces. Overcome and face those pieces one by one, one at a time.
Don't force yourself to change if you are not prepared. Don't rush also, changes need time. Don't get discouraged if sometime you fall back to old path. When you fall, just stand up again.
Don't force yourself to love a girl if you really can't love them. Let
the feeling developed naturally. You are still young, mix around with more friends. If you feel you have gay inclincation, you don't have to "reinforce" that you are gay by having a bf, sex or etc.
Perhaps one day, you will find someone who can really accept your imperfection.
Don't get discouraged also when pray hard to god and god didn't changed you much. Perhaps god want you to change yourself with determination, he can only guide you all the way.
That's all I want to say. Wish you all the best.
"you control your own destiny and how your life is going to be."

I have some additional thoughts: I always heard people ask gay people to change, but at the first place, can they accept a ex-gay as their partner if they discovered? Or are they expecting the ex-gays to marry among themselves (eg. ex-gay marry ex-lesbian). Another thought: if in another parallel world, gay is a norm and opposite sex is morally wrong, how does it feel to ask those normal people to change their sexual preferences?