I've found in my life that I can just admit that to God, that I don't feel like rejoicing but instead feel fearful or bitter and rotten or doubtful or worried or hurt or alone (or whatever the feeling is), and I can have an honest heart to heart with Him. We don't have to come up with anything on our own. We can ask Him to fill us and take over. His love and joy will start filling us up. He will uplift us! "When my cares overwhelm me, YOUR COMFORTS DELIGHT MY SOUL!" (Ps. 94) And then suddenly we feel like praising!
God is always accepting us. We can bring to Him our honest thoughts and feelings. He will never reject us. All our feelings are valid before Him. He truly understands us and our hurts and how we feel! He doesn't ever minimize our feelings or wounds or say we 'shouldn't' feel or hurt! He died for those hurts and pains, they were very real when He bore them on the cross for us!! He's taken them away and doesn't want us to bear them a minute! They are illegal, really! NO negative feeling is meant to be on us! We have been made whole and given joy and peace, and that's our rightful inheritance that He wants us to experience always. And He will make us to experience it! Not us, not by our work but His!