The most important truth I can tell you is that God knows your situation, and He does care, because you are very valuable to Him. If He didn't love you, would He want you to spend all eternity with Him? And yet He does.
You are so valuable, in fact, that if you had been the only person on earth who needed forgiveness, Jesus Christ still would have gone to the cross for you. It's hard when we don't feel well (either physically or emotionally) to think about anything else–but when we focus on Christ and His love for us, our attitude can begin to change. The Bible says, "This is love: not that we loved God, but that he loved us and sent his Son as an atoning sacrifice for our sins" (1 John 4:10). If you have never done so, ask Christ to come into your life today.
Proper medical treatment is important; don't feel guilty or embarrassed because you need it. But ask God also to help you turn to Him when those feelings of worthlessness and hopelessness threaten to overwhelm you. Prayer and the promises of God's Word can do much to keep those feelings at bay.
Some of God's choicest servants struggled with the same feelings you have, but with God's help they overcame them. This was the psalmist's experience: "Why are you downcast, O my soul? Why so disturbed within me? Put your hope in God" (Psalm 42:5).