Having these troubles has been a bane of my existence for quite a while. In my case I tend to put it down to me being awkward and useless rather than too polite or others being too rude (although to a certain extent, there are some quite assertive speakers in these circles, and to a certain extent my reticence to interrupt can be taken as a reaction against an earlier me that just didn't care that he was riding roughshod over other people's conversations, and so considered polite in some sense). I have some tricks for dealing with dud parties: the 1am rule (ie I can go home at 1am and still say to myself I gave things a chance - one of my favourite parties only really got going for me at 12ish. 1am can be earlier if the party started early.), getting involved in games, hogging the stereo, reading graphic novels. I do often go home frustrated, and I know that sometimes I can clumsily interrupt conversations (usually popping the stack by several levels) some times.

Small groups and one-to-one can help, although at various times I had a habit of being stuck as the non-participating one in three-person conversations. That could get extremely frustrating.

All that said, sometimes I do just want to be there, on the edge of things, listening in without feeling the need to actively participate. But at other times I really do feel left out.