Firstly welcome to ASPIESCentral.
Yes I have awful selective memory issues, mainly short term memory and I've learnt that it's actually a common trait of people on the autistic spectrum, I was very slow to develop speech as a child as opposed to people with Asperger Syndrome who didn't and I'm not sure whether this has any relevance or not. The problem has gradually become even worse as I've got older and I'm now 48 years old, I guess everyone's memory degrades somewhat with age, but it's probably more of an issue for myself and others in a similar situation since it was poor to begin with. I will however mention it to my doctor just in case as a few people have been nagging to do so for a while.
I have exactly the same issue as yourself, I can be about to talk about something and then completely forget what I was about to say or even what I was already saying. At least if I've already started I can usually ask the person I was talking with to remind me what I was talking about when my mind goes blank, but it's often a lot more frustrating if I haven't said anything at all yet and sometimes I will never be able to remember again. I think this can happen to anyone very occasionally, but it happens quite often to myself. I've noticed it can happen more if I'm about to say something or I'm in the middle of saying it and I'm distracted before I can say or finish it, E.g. I'm about to talk about something specific I experienced earlier that day and I'm interrupted with a simple question or instruction such as "are you warm enough?" or "can you shut the window please?" before I get the chance to say it.
Another issue that is particularly stressful is losing things and putting them down or away somewhere without remembering doing it. For instance last week I was looking for an important letter and after searching my flat for about an hour, tearing my hair out, I finally came across where I'd originally put it "safe" and I was really relieved. I then told myself that I definitely wouldn't lose it again and that I'd put it somewhere much more obvious this time, but a couple of minutes later I was cursing myself as I'd amazingly lost it yet again and I just couldn't believe it. This time luckily it only took me a couple of minutes to find the letter again as I'd put it down subconsciously nearby, but this kind of thing is very frustrating to say the very least and is happening more often for myself.
I can also forget appointments, whether or not I've taken medication and more. Remembering appointments is easily solved these days by using a calendar as long as I ensure I don't delay updating it as otherwise I can forget to do so and medication can be put into blister packs that are labelled by the days of the week (in the UK you can ask a pharmacy to prescribe them like that if you need it, I'm not sure about other countries).