I'm certainly no expert Pies, but a few thoughts came to mind when reading your post.
First of all, are you mentally focused on practicing BEFORE you actually begin? Meaning, have you mentally prepared yourself with a set goal in mind to accomplish prior to starting? I have always found it useful to begin every practice session with a mental checklist of what it is I will actually practice, what I want to gain, and thought about HOW I will practice. I get as familiar with the piece at hand as possible prior to actual hands on playing, by listening to a recording (or several), reading thru the score, reading thru while listening, and dividing it up for practice sessions.
For me anyway, if a piece is particularily difficult, and I take too much on at first, I know I tend to get frustrated which will cause a lack of interest, and loss of focus. So by choosing to learn only as much as possible at first, before even sitting down, I don't set unrealistic goals, therefore, I almost always accomplish what I set out to achieve during practice. It may be a page, a measure or only a short progression of several chords or notes, depending on difficulty. My REWARD is that by doing this I acquire a successful practice session, which encourages and inspires me to continue.
Next, do you plan your practice sessions at times when you are most mentally focused without distractions? If you are rushing between jobs or classes, and only have a few minutes, TV is on, people around, dog barking (you get the idea), it would be very hard I would think to focus. Also, are you as free as possible from stress and mental fatigue. It's hard to practice when one is tired, not feeling well, etc. I've also found that a brisk walk, excercise, shower, short nap will do wonders before practice if I'm tense and preoccupied beforehand.
I don't think penalizing yourself for making mistakes is necessarily a positive move. I think it far better to reward good results, than punish bad. By setting a small goal, and preparing yourself and the environment in which you practice prior to beginning, this should be possible.
Also, the times that you say you have really been focused and able to concentrate, do you recall what may have been different during those times? Possibly time of day? Type of piece you were practicing? Length of practice time?
Do you enjoy what you are practicing? Are they pieces that you have to learn, or do you pick whatever you enjoy? I assume, as you said you currently do not have a teacher, that you pick what you play. Is it possibly that while you like the piece, it may be too difficult, which makes it hard for you to focus, or that you are taking on too large a chunk of it at one time. It may help if you only choose 1 or 2 measures or lines before you practice next time, and decide to make that your goal for that particular practice session. Go over the piece without actually playing it. Think about how you will play it, how you will have to move your fingers, wrists, etc. and take some time to mentally prepare yourself for the practice session. Make sure you have time to practice, and are not rushed or in a hurry to do something else.
I hope this helps alittle, good luck!
O
