Let me try my best. :D

* Alternation is a bit tough. I can't think of a direct translation.. Let me try to think of it first. :D

This here is supposed to be an alternation to all my former [and further] love confessions.

Ito dapat ang magsisilbing kahalili ng lahat ng aking nakalipas at magiging pagtatapat ng pag-ibig.

*kahalili means alternate/substitute. Unfortunately, that's the best I can come up with.

I don’t understand a single word of what’s written there, but, you do. And that’s the important thing. As someone’s translating this for me, I don’t wanna talk too much

= Wala akong naiintindihan isa man sa mga salitang nakasulat dito ngunit alam kong naiintindihan mo ito at iyon ang mahalaga. Dahil may nagta-translate lang nito para sa akin, ayaw kong magsalita ng masyado.

Just this: I consider myself to be the luckiest person on earth.. cuz I got u!
Words can’t describe how much u mean to me.. how much I love u and how much I need u.

= Ito lang ang masasabi ko: Para sa akin ako na ang pinakamaswerteng tao sa mundo dahil nandyan ka.

Walang salita ang makakapaglarawan kung gaano ka kahalaga sa akin at kung gaano kita kailangan.

*makakapaglarawan came from ilarawan means to describe but it's hardly used in conversational Tagalog since it's usually considered an old-fashioned word so you can just use 'makakapag-describe' if you want

Baby... I promise, I’ll never let u down.
= Baby, ipinapangako/pangako ko sa iyo, hindi kita bibiguin.

I miss u badly and the only thing that keeps me up is the thought of us meeting soon..

Formal:
Labis na akong nangungulila sa iyo at ang tanging bagay na nagbibigay ng lakas sa akin ay ang kaalaman na malapit na tayong magkitang muli.

Casual:
Sobrang nami-miss na kita at ang tanging bagay na nagbibigay ng lakas sa akin ay ang kaalaman na malapit na tayong magkitang muli.

* Nagbibigay ng lakas literally means something that gives strength but it's the closest thing that I can relate to 'keeps me up' since there is no direct translation for the phrase.

Wanna use this opportunity and say, thank you! For everything..
= Gusto kong samantalahin ang pagkakataong ito para sabihing salamat para sa lahat.

* That's it. That's a sweet gesture on your part so good luck on your confession! ^_^