One lesson with a great teacher could probably solve this more quickly than 10 years of trial and error. As Manny noted there are equpment and playing based problems. If your equipment is OK (something that we cannot analyse over the internet as there are lemons even with pro horns) then it is a playing problem that could be breathing, ear, embouchure or any combination of the above. With all the recommendations given here, you could spend weeks, months, years searching for something that a teacher could point out in 1 hour! Playing in tune starts with a stable playing concept on reasonable equipment and the necessary amount of "ear" training. If one of the factors is deficient, you can always twist the other 2 to compensate - the results are not "optimal" however.