The context of this scripture (1 Timothy 6:12) is not a position of a mental or emotional failure or because you are weak, rather the apostle Paul is reminding us of our permanent security so you do not have to follow after the patterns of this world (or the lies about ourselves and wrong spiritual formulas).
"It's not a 'works' fight, it's a 'focus' fight."
Paul is reminding us we have a WHOLE new identity (not of weakness) and because of this we can "fight" to stay focused on the eternal security we already have ... even in depression, anxiety, or any other challenge. It's not a works fight, it's a focus fight. Because a great focus always leads to new ways of living, not vice versa!
Paul follows up saying "fight the good fight" by saying "take hold of the eternal life to which you were called." This means your true identity and grace in Christ is already within reach ... for any journey, condition, or circumstance. So, “fight the good fight” really means …
Don't give up or give into anything contrary to what Christ has already fulfilled in you! Despite the condition (depression, anxiety, or other challenges) take hold of (focus on what you do have) and rest in your new creation reality!
Let's review some scriptures just to highlight who we really are.
-- Your breakthrough in Christ has already been fulfilled ... Jesus already made a breakthrough once and for all, we are completely new and lacking in nothing, and no longer need to strive for more of God's blessings or more spiritual breakthroughs. We are living out and growing from our new wholeness in Christ (Romans 6:10-11; 2 Corinthians 5:17; Ephesians 1:3 and 2:14; 1 Peter 3:18).
-- The wholeness and oneness with God is fulfilled, there is never any distance or separation ... Jesus is at rest with us and we are at rest with Him; we are complete and have oneness in Him; He has already made us secure over-comers through faith; resting in our eternal security; no more striving to achieve more faith and freedom (Romans 5:1-2; Ephesians 2:13-18 Galatians 2:20; Colossians 1:27; Hebrews 4:11; 1 John 5:3).
-- His grace works within us for our benefit (Growth) ... We rest in knowing He is guiding us by grace and working in us toward His good advantage, will, and pleasure; no need to strain ourselves to get more grace or blessings (Romans 8:28, 32; Philippians 2:13).
-- "I'm satisfied with you" ... Christ is already our security as the author, perfecter, and finisher of faith and our lives ... He cares for us through all of our afflictions and trials; no need to achieve or impress Him to be free from our painful circumstances (Philippians 1:6; Hebrews 12:2).
This is what we "take hold of" and rest in through the ups and downs of depression, anxiety, etc. God has given Himself for our journey -- our weaknesses -- a grace sufficient for every part of life that feels like a thorn. If God said He will be with us ... I think He means it. Depression, anxiety, or other similar challenges is not a sign of God's absents, it's just the toils of this life, but we have one greater in us than in the world, working toward our advantage, good things (pleasure).
Jesus fought the good fight in secure rest, not insecure tests ... for more works!