We'll support you of course, but please remember that what decides things in the end is your own passion.
Literal
We [topic-は] of-course support-do [but-が] last [at-に] decide [nominalizer-の][topic-は] your passion [copula-だ][quotative-ということ][object-を] remember-[progressive-ていて]-please.
熱意 'passion, enthusiasm.' 最後に決めるのは 'what decides in the end is' — a cleft sentence structure. ということを覚えていてください 'please remember that' — ということ nominalizes the whole preceding clause. Motivational but realistic advice.