'I've' is short for 'I have'. You can only used the present perfect when you are talking about incomplete actions. The simple past is used for complete actions. I haven't been stressed out since I've worked at the company. < Here you use the present perfect, so you insinuate that you are still working at the company. The use of the present perfect means that this action is incomplete. I haven't been stressed out since I worked at the company. < Here the simple past means that you have finished working at the company. The action is complete. Likewise: I haven't been poor since I've worked at the company. < Here the use of present perfect means that you are still working at the company. I haven't been poor since I worked at the company". < Here the simple past means you have finished working at the company. The action is complete.