You can train your voice to change. I have heard that two times in my life. Take lessons, most singers have.

Buy a cassette tape with "Vocal Eases." These are exercises that are designed to expand your range and flexibility with your voice. I used these and voice lessons to improve my voice. Also, join a choir if you can. Singing with others can really boost your confidence.

Concentrate on your low voice for now and move high through slides. The reason you sound like you are singing through your nose is that you are trying to produce sound from the soft palate (the back part of the roof of your mouth). You can actually get the sound from your vocal chords if you do some exercises and become aware of the palate production. Lean your head back and drop your jaw. Do some scales while flexing your tongue back and forth. This may keep you from resorting to palate production and help you stay "in the chords." Sing while running or flat on your back doing gentle spine turns on the floor. Get a physio ball and roll on it on your back while producing sound. Do groans and growls as you move through the scales. Avoid forming vowels in your mouth. Avoid singing with amplification and hearing yourself "second or third hand." Do not tune with electronic media. Use a piano or, even better, a violin or cello string. If you can work with a cello for practicing slides and ear tuning, that would be great. Remember, the vocal sound production of many popular artists is very poor and is often covered up by editing. They are singing from the soft palate and limiting th years that you will be able to sing. You might be a much better singer than you think.

Sounds like you are singing with your throat muscles rather than your diaphragm muscles. Learn how to sort-of push the notes out with your diaphragm muscles. As you go up in pitch, try to make it feel like the sound is coming from the center of your chest. As you go higher, the center of sound will try to rise, too. Hold it down as long as you can, without letting it rise into your throat. Sooner or later, it will, but the more you practice, the easier it will get. Saying ha,ha,ha,etc., in a lound, deep voice will help strengthen your diaphragm muscles some, if you do it several times a day for several weeks.