I'll never max mine either. <garrus>And I'm okay with that</garrus>.
I unlock a lot slower than most of BSN because I don't do Platinum and have zero interest in being backpacked through it just for credits. Playing the game at a level I enjoy and can contribute to is more important than virtual shiny things. With all the Rares at X, my manifest is what one might term "functionally maxed" anyway, if one considers URs to be bonuses.
I have recently maxed my manifest, with very little platinum. Methinks you will get there too.
The key for me was to find friends to play with, at about my skill level. Matches go by much faster with competent teammates who play with you, and not to beat your score, while wipes become rarer and rarer. Also very important, you end up needing significantly less consumables, reducing the need to sink credits in those. And finally, the game remains fresh and fun, so you might find yourself playing more than if you were just pugging 
That's not to say that your philosophy of playing for fun and feeling like you are contributing isn't right! I just don't think that that, and a maxed manifest, need be mutually exclusive 