Forgot to Remove a Dissolvable Stitch After Breast Lift?

It will be best for you to call your plastic surgeon's office and see how he wishes to handle your specific situation.  Remember that he is ultimately responsible for  the results of your surgery;  you may receive conflicting/confusing advice online.

Best wishes.

Hard to make any clinical recommendations without photos or an exam. You are
going through an active healing process that will take months to
complete. Most plastic surgeons put
specific sutures in certain tissues for a reason. In a breast augmentation and mastopexy, most
usually close the tissue in layers to keep the tissue together and to
take tension off the wound. Deep layers are usually absorbable and the
skin sutures non absorbable that have to be taken out. Sometimes
sutures from the inside will spit out through the wound. If this is the
case you may have a little opening. This is usually OK, and will heal,
but should be seen by your plastic surgeon or whoever is covering for
him/her.  I tell my patients... No kitchen surgery, come in and we will take
a look. Please continue to inform your board certified plastic surgeon. 

My recommendation is to trim it or remove it.  In order for your body to dissolve a stitch, inflammation is required.  The more inflammation, the longer it takes your scar to look good.  If it is a lot of trouble to go back, don't worry, it will likely fall off without consequence.