Reusing part of my thesis within the same thesis
Yes, of course. There are no issues with this at all. Self plagiarism only occurs when you don't cite your own words from other publications. Within a single document there is no problem. The reason that we self-cite is to let readers easily find the original source. But that doesn't apply here. 

OTOH, make sure that it doesn't read "awkwardly" when you do this. It is just a style consideration, though. 

I'm assuming that your abstract is actually part of the thesis, say a preamble. 

However, if the abstract is to be published separately from the thesis, you should cite the thesis in the abstract. But you would do that in any case.