We have several concepts for pitch. They all need a certain degree of familiarity before we can be "successful".
Playing alone with or without a tuning device is dangerous as we do not play "well tempered". The tuning device is to get our concert Bb "in the ball park". If we play to all the other notes, they will never sound right.
Our performing pitch is always relevant to something else - it can be the last memory, or it can be another instrument or recording.
In addition, if we are playing with serious upper body tension, the pitch will sound sharp even although a tuning device would measure just fine. If we do not have enough tension, the sound is dull and it sounds flat although it may measure OK.
If you are not taking regular lessons, then just play along with recorded music. That can be a a CD with concert band music or marches. It can be church hymns or jazz standards. The important part is that you rediscover having your pitch relative to things around you (accompaniment) as well as playing enough to get a good resonant sound throughout the registers that CAN sound in tune.
Your actual problem is the ear/brain not talking to the body/lungs/embouchure, or the body/lungs/embouchure not reacting predictably to the signals coming from the brain. If you had these skills at one time in your life, then a lot of the work is already done. Simply play enough and those skills will move to the short term memory and become more automatic.