I sympathise, Daisy - it's happened to me too, and I should know better.
It can be annoying working in a word processor first because all you have is your words; you have to add all the forum formatting, links and images later.
Try this while typing in the forum reply box: Hit Ctrl-A (select all) and then Ctrl-C (copy) from time to time, and especially if interrupted or before you preview or post, and make sure you keep redoing it after you copy & paste anything else. If you can do this as a matter of habit it saves a
lot of frustration.
By the way, paste is Ctrl-V. And on a Mac substitute Cmd for Ctrl.
Ha! I just lost this one and got it back cos I'd followed my own advice!