As far as sticking with pieces... The main thing that deters people from sticking at something is a lack of progress. If you can't see any light at the end of the tunnel then it seems fair enough to bail out and try something else. The problem with this is that it doesn't address the reasons why you were unable to make substantial enough progress. If you develop good (efficient and effective) practice habits then progress becomes automatic (not necessarily always incredibly fast, but always moving in the right direction), therefore if you are constantly moving towards your goal then there is no reason not to stick at a piece. This is especially true since the slowest progress is usually at the beginning, and after the initial labour things often come together very quickly and easily.
As far as Beethoven sonatas go... I would resist the urge to dip into bits and pieces of different sonatas, since you won't have a clear goal in mind and it will be very tempting to keep chopping and changing. Instead, pick a sonata that you really love and think is manageable (op.49 no.1 or 2 would be good starters I think) and COMMIT to learning it. Not giving yourself a get-out clause is a great way to stay motivated!
Edd
Thank Edd! I find this really helpful as well and I also understand what you mean in the first paragraph. Maybe then I should probably try and finish the Presto Agitato

as all I need to do is learn the rather annoying 'chordy' parts. But I do also like the Andante of the Op.49 no.1, and would be a good start as I already know how to play Op.49 No.2. As well as the Tempest...ahhhh see this is also a problem, I like too many of the Beethoven sonatas and never manage to stick to one cause I end of drifting toward another.

Im glad you found it helpful. I think the key is you pick one sonata and learn it. Just avoid the late,r bigger, technically challenging sonatas like the Apassionata and such and finish the Tempest, Moonlight, and shorter earlier Sonata. I think you should try and find a way to set a date to perform the piece in some context that may help give you the motivation to learn it thoughly and memorized.
This is actually a real good idea with setting a date, and I also understand fully what you mean about not learning the harder sonatas, I think this will give me a bit of elimination of choice when choosing the sonata to learn.
What would really help me though, and I would be ever so grateful if you could, but could you from, say 7th Aug 2011, set dates for me to learn
-Tempest (I know most of the 1st mov, and just before things go slightly 'alien' in the 3rd so about 2 mins)
-Moonlight (I know the 1st mov. already, and most of the 3rd)
-Op.49, No.1, nothing... >.<
I really appreciate this advice as well so please get back to me! :

Cj