Maintain erection. Please advise.

Hi I was in relation since 2011 to 2016. I was good and very active person . I used to get very strong erection at any point of the day. However I had a relation break up and also was in depression few months for which I took anti depressants tablets for 6 months. Then I was better and normal , also discontinued the anti depressants tablets. I also work as IT professionals, due which I have mild backaches due to my job nature (sitting for hours). But its mild pain. Now my issue is 1.I dont get erection consistently daily morning,which I used to get 3 years ago. 2. When I'm having sex, my erection doesn't sustain or softens (or I can say maybe the erection reduces) if I stop for few seconds or change position from missionary. Please advise. Thanks