I don't see the value in staying on 20mg. Imo, your GP should be thinking of increased the dose to probably to 40mg after a couple of weeks on 30mg. The alternative is to switch to another AD. Citalopram (Celexa) and the very similar escitalopram (Lexapro) have a good record for working when another SSRI has failed.

Does your GP know about the agitation and the suicidal thoughts? If not s/he should. Also ask for something to ease the anxiety. White-knuckling it is counterproductive.