A semitone out is a lot. If you are practicing scales with intonation and finger placement specifically in mind and still regularly being a semitone out then my first suggestion would be to put those tapes back on. If you never had them on in the first place that might be a contributing factor.
The second thing I would point out is that practice, to be of value, needs to be mindful. By that I mean that before you start your practice you have to have a plan. You know what you are going to try and do and how you are going to do it. While you are practicing your brain is fully engaged. You are paying attention to what you are doing and correcting and adjusting as you go.
The third thing I would point out is that improvement is gradual. It is not, unless you are lucky, an overnight thing. It happens slowly over time with much mindful practice. Good intonation should always be at the back of your mind but you should not stop everything else. Repertoire and etudes should still be part of your daily routine.
One addition to your scales would be to switch on a tuner while you play and adjust your finger positions with the aid of the tuner. Unless you have already mastered another instrument it is very likely that your ear will require training just as much as your fingers.
Finally, by insisting to try and learn without the help and guidance of a teacher you are metaphorically tying one hand behind your back. If you paid money for your instrument then it will be very much worth your while to pay money for instruction.
