Something I like about Karate is that you can do it 100% alone all the time and you don't need any space. One move you can learn to do all by yourself is to hit very hard. Karate focusses on the reverse punch, if you look at the makiwara (punching plank/pad) training they do in traditional Karate (which was considered to be a full1/3rd of training!) If you're someone genuinely afraid of getting beat up and you want to start doing something all on your own start hitting something - start safely, gently, but build up to hitting very hard; harden your fists; inure your spirit to hitting things very hard. You can't learn to or practice grappling by yourself, but you can indeed learn and practice hitting hard. This won't make you a "boxer" who fights tactically, but it will give you a very important skill that could scare someone off or actually hurt them if genuinely attacked. You could also kill someone particularly if you're big, so don't be dumb.
Otherwise, quick physical skills that you can develop on your own to help you are jumping and running. If nothing else, these have a lot to do with just jumping away, not falling down - so keeping balance in a dynamic environment - some cardio-stress training for when your heart starts to beat out of fear, and running off.
If with a trainer - get some boxing gloves on and spar, learn some throws/trips and wrestle around, learn some pins and try to pin/escape and stand up in a ground wrestling situation. People can learn this stuff pretty fast to a basic level.
Definitely the end goal should be to run off.