Some of this can be mitigated by transposing to a lower key, singing alternate melody lines in the trouble spots (many aging vocalists do this successfully), or getting some foxy backup singers to take the pressure off and enhance the performance (remember, we're entertainers). I feel for you, though. Always envied vocalists. Personally, I've benefitted from the aging process. What was an annoying nasal whine back in my 20's is now a gruff baritone much more suitable for vocals (despite a limited range). Don't give up, just build up slowly so you don't damage your voice, and use what you have to its full advantage. Find material that lends itself to your new range. This will also expand your catalog and consequently make you a better bassist. Hope this helps.