Just keep going. Always have your best stuff equipped when you go to the Cryptarch. You'll get it eventually.

When you decode engrams, do one at a time, equipping the best light each time. Infuse your legendaries with whatever us better after, takes about two weeks of light play to get up there once you hit lvl 40. With the other characters, use them to decode their legendaries, but any blues or sister legendaries, use your highest and infuse. My titan was 288 before I even used him other leveling to 40 Lol