I run into this with what seems like most characters, but the three that come to mind are Millia, Sol, and Ky. They just corner me and continue throwing out quick attacks, and I'm kind of at a loss. It feels like their attacks put them at a relatively safe enough distance for them to just continue the onslaught, but I think it's mainly because I'm not used to the faster pace yet and don't really know all my options.
When I was first learning the game, I took advantage of absolute guard a lot. Absolute guard is where if you are in blockstun, you will continue blocking whether or not you are actually holding back. (You still have to account for lows and overheads.)
Here's something you might want to try, if you're playing a long set with someone or in some other situation where losing is ok. If you're not sure about where gaps are in a blockstring, hold forward where ever you think a gap may exists. If you continue blocking, then it's a solid blockstring. If you get hit, then there is a gap. And where ever there is a gap, you can backdash, reversal, or throw.
Leo's backdash is kinda bads, but he has his flashkicks and his supers to use as reversals. If you wanna experiment with how big the gaps are, Leo's 2P is his fastest button. (5f startup)
edit: Also, generally, blockstring chained into an overhead, there is a large enough gap for anyone to attack out of. Unless of course there is projectile coverage. Chains to overheads without a large escape gap are usually extremely punishable on block.
