Coming late to the discussion, and I don't have much to offer by way of mixing, but I hear some things in your vocal technique that are causing the pitchiness in some places. Basically, you are using only half of your voice to sing with, and you need to have both registers equally strong. You have the lower register, or "chest" voice - which is what you're singing with currently, and there is falsetto, which is the upper register, or "head" voice, which is very weak and not participating at all right now.
The lower register and falsetto work antagonistically, and so if they aren't equally strong and coordinated, one will take over whenever it's present. The way you are compensating for lacking a developed falsetto is you sing nasal as you go higher. Ideally, as Broken_H stated earlier, you would need a good vocal teacher to help you through this. However, there are a few exercises you can do that will get you started:
Always warm up this way: start out in chest voice, at a low/comfortable pitch. Sing a bright "eh" vowel, and be sure to make it sound "manly" (for some reason, this helps women bring out their chest voice LOL). Also be sure to sing at a comfortable loudness. Soft will cause the chest voice to not participate. So sing "eh" and hold it until you are just barely out of breath (don't go too long so that you end up squeezing). Chest voice should feel, firm, but the air should feel like it moves freely. Be sure not to squeeze the throat when you sing. If you feel a squeezing sensation, then try to make the sound darker (lowering your larynx) and increase the air flow.
Repeat this going down by half step, singing one note at a time, until you reach your lowest note. Then proceed going up by half step, but stop when you feel you are straining/squeezing to reach the higher notes in chest.
Then work on your falsetto: This is a dark hooty sound like an owl. "Dark" means the larynx, or your voice box, lowers. If you place your fingers on your larynx and yawn, you can feel it go way down. That is too far for singing, but that will show you which muscles to engage. So sing a higher note (C4 is a good place to start) on "oo", holding it out as long as you can comfortably, and as loud as you can comfortably. Depending on how your voice is, this may be very difficult for you and make only come out squeaky. Try to work down by half step, making the sound as dark as you can. The air that you use for falsetto/head voice is a lot more than you use for chest. Take this sound down as low as you can, but don't let it become chesty. Then go up by half step and go as high as you can without straining.
Once you do this, you can go back down into chest and sing on a single pitch a combination of vowels, like "eh-ah", one note at a time. Or you can do a E-D-C pattern on "ah", going down by half step. Continue alternating between chest and head voice exercises like this for 2-3 times.
The key is very simple, concentrated exercises to focus on the feeling and sound you are making. A good teacher can guide you to the correct sound, but this may help.
Explaining this stuff on a forum is really tricky, so I hope this made sense. Please let me know if you don't understand something I've said.
