I find this really difficult too but the advice I was given is to manually focus on something and then replace the something with yourself. Good luck!
Might be worth investing in a cheap remote release.
I've got a really cheap Yong-nuo one, which instantly solves the problem because, unlike self timer, it focuses just before it takes the shot
this, however my cheapy one only focuses some of the time.
Think it cost me about £3 off the bay![]()