You don't need a vasectomy. You need to see a psychiatrist to get to the root of why you feel the way you do.

I went through the "fix" 12 years ago. Never had it impact my desire or passion. Are there other things going on in your life that would cause depression (lack of desire or lack of interest in things you used to enjoy is a common symptom of depression)

You said you made the decision four weeks ago...I'd wait a year if I were you.  Make certain you want to do it.  But as the others said, it's not going to make you feel any better mentally.

If you do have it done it's no big deal really.  I had mine done at 46.  No issues.  Nothing to it.

Good luck.

I was 35 when I had it done.  I have no problem getting aroused with a woman I'm attracted to.  I still crave sex and all the fun that goes with it.  While I do not have a gf right now, it's nice to know that if I did and we were doing bareback sex, I would not have to worry about her getting pregnant, if she were still fertile.  Of course, covered sex with providers makes that thought unnecessary, but it's nice to know anyway.

Swim

What you are describing is exactly how I was feeling.  I got my testosterone checked and it was quite low.  Testosterone replacement therapy made all the difference in the world.  1 week into the therapy, all of a sudden I was a raging horndog again --- er, let me rephrase that.  1 week into the therapy my desire to make mad passionate love to any female with a pulse who said "yes" returned.  

More philosophically, this experience showed me how much a chemical imbalance could affect my personality.  I just had no libido.  It wasn't like I stopped thinking women were beautiful.  I just lost the desire to have sex with them.  Get my testosterone levels back into the normal range and my libido sprang right back. (nudge, nudge)

I'm not trying to psychoanalyze you, but I suppose you might want to reflect on this question: Was there something in your erotic mind that tied your sexuality in with the ability to make a woman pregnant? Now, this doesn't mean that you secretly did (or do) want children, or that you made a huge mistake, but it is quite common for men to tie their potency/ability to impregnate with their sense of self in a sexual way. If that's the case, you may either need to think about reversing it, or finding a way to tap into that same thought pattern while engaging in a little suspension of disbelief.

Also, you could just need some time to come down and refuel. You did just go through a medical procedure, and medical procedures are usually very unsexy. So give yourself a break, and give yourself permission to heal both physically and psychologically. Boners, after all, are brain dependent!