First of all, welcome to YC!
If I understand well, your problem is that you cannot express your emotions through music, and don't know why.
Imagine, that you want to write a love poem in Spanish, but you know nothing or only a little about the Spanish grammar. Well, music is some sort of language, and if you don't know its grammar you can't express yourself through it. ( <-- it is my personal opinion, many have different thinking about this, but please, do not start an endless "What is music?" thread again. We are here to help, thank you.)
Ok, here are some practical suggestions how to improve your melodies:
1. Learn rhythm and meters. Rhythm is the primitive level of music, it is vital to understand different note values and meters.
2. Learn scales, minor and major at least. Learn key notes like tonic/1st, dominant/5th, subdominant/4th, and tendency tones (leading note/7th , supertonic/2nd, ...). There are stable notes and less stable or unstable ones, the latter need some sort of resolution.
3. Learn phrasing. Make short phrases instead of a very long melody line (In other words, try using "simple sentences" instead of one long "compound sentence"). Try to build short phrases purely by different rhythm values / or purely by different pitches with same note values.
4. Learn cadences. Listen to the difference between ending on the 1st degree of the scale and ending on the 2nd, 3rd... etc. degree. Experiment with cadences and listen how they sound.
5. Study scores of great composers. Listen to parameters listed above (1-4.)
Good luck!
Máté