What is the point of working on my marriage if my partner isn’t doing likewise? Doesn’t it take two people to make a relationship work? Should I bother to seek individual counsel and direction?

Two Men Courting Girl’s Favor, Norman Rockwell, 1917
Remember that you create half of the relationship. Anything that you do better, in either giving or receiving, can only improve the situation. Of course, you cannot arrive at a consummate marriage, a reciprocal union, from only one side. But you can move towards it. Your healthy human needs will not be adequately addressed by your stagnant partner. (This is a problem that needs a separate discussion.) But you can grow in wisdom and love and partially reshape a relationship that will then look and feel different to both of you. Your personal growth continually renews and updates the invitation to your partner to join you in a dialogue of eros.
The soul purpose of life is to grow in capacity to love. Philosophy and psychology alone will not get you there. Love is too big of a mystery to tackle in the abstract. A bottom-up approach is needed. Start with an example, not a concept. Perhaps this soulful purpose adds to the basic desire for romantic coupling. Grow to love this specific, flawed, frustrating, infuriating, ordinary person in front of you. If you can love any one particular person unconditionally, unrestrictedly, unreservedly, then you can love anybody. Then you will know a greater love than connects all things. Ironically, what seems like an individual, one-sided endeavor actually helps you participate in something even more inclusive than your marriage. So, yes, working on “relationship issues” by yourself is still worthwhile.
In this sense, it does not absolutely matter who that specific person is. The beauty and the suffering of marriage is that any person, fully revealed, is difficult to love. Some are more difficult than others, of course, and we all might prefer a shallower learning curve. Since perfect love is not attainable and all are equally worthy of love, you are ultimately tackling the same project with whomever you choose; it just has a different shape. And it’s the same project no matter how many times you choose.