My guess is:
1: The song you are playing and trying to sing is outside your key.

2: You are tone deaf.

Solutions:
Problem #1
A. Go to the BEST (most expensive) local music store, bring your "ax," explain the problem and sing your range, download from the "magical musical machine" (these things are truly wonders-entire scores are printed out in ANY key) the song within the key that the "professional" (usually a college student) suggests. Try the chording RIGHT ON THE SPOT! Try another if it does not work. (It is their "nickle" not your's)

B. Go to the "Net" try all sorts of different combinations of keys (given that You CAN play in different keys)

C. Get a "Capo" and bridge it on down....somewhere you WILL find your key.

Problem #2
GIVE UP....Take up Knitting....or shell collecting...(My grandfather, once gave my aunt lessons from a "sing-teacher" who "guaranteed" that he could teach ANYONE to sing...(He COULDN'T) the money was refunded.