Taking more than one SSRI at a time can be done but will need to be cleared with a doctor first. This can be a GP or a psychiatrist. Best of Luck!
First, is you doctor an experienced psychiatrist or a general doc? The former do this for a living; the latter do it occasionally and not very well. The usual reason for adding wellbutrin to an ssri is if you're having weight gain or sexual problems from it. Wellbutrin, because it's one of the most stimulating antidepressants available, isn't usually used for people whose primary problem is anxiety, though there are always exceptions and often the root problem is depression, not anxiety. But if the Zoloft has been working for 29 years and isn't now, it's pooped out or needs a higher dose. Adding a drug to one that isn't working doesn't do anything to help the old drug spring back to life. So if that's what you're saying, that a drug you've been taking that long no longer works, that's common and the remedy is to stop taking it on a taper that is as slow as you need to prevent serious withdrawals. Then you try something else if you still want to rely on medication rather than other means of treatment.