Venlafaxine withdrawal to sertraline instead?

Sara, I would not advise switching to another AD.  First of all, you are in withdrawal from tapering too fast and too steeply.  150 down to 37.5 mg in four weeks would leave anyone feeling the way you are!  Taking another AD to fix withdrawal simply won't work; the only thing that will fix Effexor WD is to updose your Effexor.

I would advise going back to 75 mg and see how you feel there.  If you can tolerate the level of symptoms you are feeling there, stay there for a couple of months before doing anything else. 

I imagine you were going off Effexor because either the side effects were intolerable, you felt you were well enough to come off, or it quit working.  No matter the case, you must come off more slowly.

The reason is this:  While on Effexor, your nervous system made oppositional adjustments to bring about homeostasis.  When we take these drugs, they actually CREATE a neurotransmitter imbalance that the nervous system is not happy about, and so it remodels, down-regulates, to bring about balance.  It is these oppositional modifications that then are left behind as you drop the drug too quickly, causing imbalance AGAIN.  This is a drug-created imbalance.  Your decreases have gone faster than your system can keep up with, and this is why you are feeling so awful.

People get confused and think they need another AD because along with the physical nastiness, there is also emotional nastiness caused dropping too fast: depression and anxiety are part of this, but everyone thinks it's a relapse, proof that you NEED to be on an AD. This is not the case; these symptoms are caused by coming off too fast!

So, your best bet is to back up and start over.  See if 75 alleviates most of your symptoms.  That still means a 50% drop from 150 that your nervous system will be scrambling about.  So, hold at 75 mg for a good couple of months to allow your nervous system to get caught up.

It takes four days for a drug change to come to a steady state in the blood, so give it a week of this reinstatement to see if you feel better.  Four weeks of reduced drug has led to some system recovery, so you don't want to go back to 150 as that will overshoot where your nervous system has gotten to.

Now, I will say that Ven withdrawal is some of the nastiest out there.  There are some people who can come off of it no trouble with the type of tapering you did, while others suffer greatly and end up back on the drug or on another one.  The difference is not that YOU need the drug; we don't really know why other than that everyone is different.  You are sensitive and will need to go slower.

I, myself, am tapering slowly after a failed attempt to come off ven.  I've learned a lot about what is happening with the nervous system while on the drug and while trying to come off.  I was on for 12 years.  The longer you've been on, the slower you must go.

Please see the 10% topic in this thread:  https://patient.info/forums/discuss/depression-resources-298570

I was in hell when I came off, reinstated 10 months later.  I feel great now and am down to 24.5 mg.  The idea of the slow taper is to keep you functional as you come off.  There is no benefit to coming off faster; it's not about getting the drug out of your system but about the modifications left behind BECAUSE OF THE DRUG.  People who come off cold turkey or too quickly may experience protracted withdrawal for months and even years.  It is better to get most of the healing done slowly as you reduce so that when you come off the last bit, there's very little adjustment the nervous system needs to do.

I hope this helps :-)