Putting on steris one handed is rather tricky as it is very difficult to get the wound closed properly. If the cuts are bad enough for steris, it is probably best that you get your arm checked either by your practice nurse, or at a walk in centre.
Sorry that things are feeling bad at the moment, has anything set these feelings off?
For the wedding you are going to, are you able to wear long sleeved gloves, or a shrug type cardi over the dress? Could you put a tubi grip over the cuts for your injection?