Annah, when you say "incessively invading thoughts" are you referring to thoughts of catastrope, panic, etc.? If so, you may have an anxiety/depression issue. That often requires some medical help to deal with, but you could try to do some Cognitive Behavioral Therapy on your own if it's not that bad. The site
Home - Living Life to The Full gives some good information, or you could do a search on Metafilter for more.
If you just have a hard time meditating, well, my meditation instructor said that her mind literally SCREAMED at her for months when she first started. The trick with an uncooperative mind is to let it do its thing, but don't pay it any mind
