I would make three suggestions -
1) if you are writing melodies, get away from the chord entirely as you write them - I always tended to sit at the piano and play chords in my left hand and melodies in my right, as I composed - this became deadening after 20 years or so; now I frequently work on the melody first, and worry about harmonization later - it tends to reduce my dependence on old habits -
2) a great way to harmonize things is to think in intervals - take the note "C" - this is the ninth of a Bb chord, the dominant seventh of a D chord, the major 7th of a Db chord, etc etc etc - this tends to help pull one away from strictly diatonic thinking - but also remember that a chord may work in isolation but not relative to that which precedes and follows it -
3) compose on some kind of synth, one with good sustain - this has helped me a lot - the piano, which I've always composed on, sometimes dictates its own melodies by it's very specific tonal properties - as any instrument does - a new instrument opens up the mind, presents new sonorities, new kinds of sustain (love my little Yamaha) - and will likely open up new melodic horizons -
4) listen to Mingus - incredible harmonic transitions, unafraid to follow the logic of a melody even when it leads to unfamiliar and even apparently illogical harmonic territory - and Jaki Byard - another master at violating conventional harmonic wisdom yet preserving inner logic; both of these amazing musicians created their own frame of reference (as did, of course, Ellington, Tatum, Monk, Bud Powell, et al)